revert: remove the stream throughput watchdog and the live tok/s working line (#1759)

[code-yeongyu]

Fixes #1759

Summary

Removes both halves of #1743 - the provider stream throughput watchdog and the live tok/s segment on the interactive working line - so that neither feature leaves a trace in code, settings, docs, the changelog or the tests.

Why

• The floor fails healthy turns. A stream measured at 6.1 tok/s over the 20s window had its request aborted mid tool call, surfacing as a failed tool call plus the raw watchdog string. Thinking-heavy models and gateways that batch several tokens into one delta routinely sustain rates under the shipped 8 tok/s floor, so the guard converts a slow-but-correct answer into a hard failure and spends a fallback hop on it.
• It cannot be softened where it is produced. The wording is simultaneously the retry classifier's token (packages/ai/src/utils/retry.ts), and the verdict aborts the request controller, so the partial answer is discarded rather than delivered slowly.
• The readout has no remaining purpose. It existed to make the verdict observable while it was being measured; end-of-turn rate is still reported by the builtin TPS extension.

What changed

Reverts ced1dadeb1 and d04674187c (#1743) plus e5ab39dba8 (#1748, the test stub that existed only to feed the tok/s reader):

• deleted packages/agent/src/stream-throughput-watchdog.ts, packages/agent/test/agent-loop-throughput-watchdog.test.ts, packages/coding-agent/test/suite/retry-fallback-throughput-degraded.test.ts;
• AgentLoopConfig.streamThroughput / AgentOptions.streamThroughput, the index exports and the agent-loop reader wiring are gone, leaving the start-bound and idle-bound guards as before;
• isProviderStreamThroughputDegradedError, the "provider stream throughput degraded" retryable alternation, the _handleRetryableError branch, the stream_throughput_degraded event and its notice box are gone;
• retry.provider.minThroughputTokensPerSecond / throughputWindowMs / throughputGraceMs removed from the settings type, SettingsManager, core/sdk.ts, the test harness and docs/settings.md;
• the working line is back to Working (<elapsed> • esc to interrupt);
• the CHANGELOG bullet and the three changes.md sections are deleted rather than annotated, per the issue: this is a withdrawal, not a documented change.

Stall surfacing (#1740 / #1744) is deliberately untouched - silence is a different class with its own bounds and copy.

Verification

• 18 of the 25 touched files are byte-identical to the pre-fix(agent): fail over provider streams that trickle below a throughput floor (#1739) #1743 tree (62c03f1b37); the other 7 differ only by unrelated later work (fix(coding-agent): explain provider stream stalls instead of showing the watchdog string #1744 stall copy, the resident-store change, /rename), with zero throughput-related diff lines.
• git grep -E 'minThroughputTokensPerSecond|throughputWindowMs|throughputGraceMs|StreamRateMeter|formatWorkingRateSegment|StreamThroughputDegraded|stream-throughput-watchdog|streamThroughput|estimateStreamedUnits|stream_throughput_degraded|getWorkingTokensPerSecond' returns nothing.
• Focused suites: logs posted below.

---

Summary by cubic

Fixes #1759 by reverting the stream throughput watchdog and the live tok/s working-line segment from #1743, so slow-but-alive streams finish instead of being aborted mid-answer. The 8 tok/s floor was tripping healthy turns from thinking-heavy models and batching gateways, and the abort discarded the partial answer; the rate readout existed only to make that verdict visible.

What changed

• Deletes the watchdog module, its error type, the retry classification path, and their tests.
• Removes streamThroughput and the retry.provider.minThroughputTokensPerSecond / throughputWindowMs / throughputGraceMs settings; existing values in user configs are now ignored.
• Restores the working line to Working (<elapsed> • esc to interrupt).
• Removes the docs rows and changelog entry, and updates fork-tracker notes to record the withdrawal.
• Leaves silence-stall handling untouched.

^{Written for commit fedd1ab. Summary will update on new commits.}

[code-yeongyu]

Removal evidence (measured)

Tree state — reverts ced1dadeb1 + d04674187c (#1743) and e5ab39dba8 (#1748) at HEAD 73bd1577c2.

• 18 of the 25 touched files are byte-identical to the pre-fix(agent): fail over provider streams that trickle below a throughput floor (#1739) #1743 tree (62c03f1b37). The other 7 (packages/ai/src/utils/retry.ts, agent-session.ts, interactive-mode.ts, three changes.md, CHANGELOG.md) differ only by unrelated later work - fix(coding-agent): explain provider stream stalls instead of showing the watchdog string #1744 stall copy, the resident-store change and /rename - with 0 throughput-related diff lines against that base.
• git grep -E 'minThroughputTokensPerSecond|throughputWindowMs|throughputGraceMs|StreamRateMeter|formatWorkingRateSegment|StreamThroughputDegraded|stream-throughput-watchdog|streamThroughput|estimateStreamedUnits|isProviderStreamThroughputDegradedError|stream_throughput_degraded|getWorkingTokensPerSecond' on the branch: no matches.

Focused suites — run on a second machine (macOS arm64, bun 1.4.2, vitest 4.1.11) from a worktree checked out at 73bd1577c2 (HEAD asserted equal to the pushed branch before running):

package	files	result
packages/agent	agent-loop-stream-start-timeout, agent-loop-idle-timeout	6 passed (1 file)
packages/coding-agent	interactive-mode-status, interactive-mode-working-status, settings-manager	168 passed (3 files)
packages/ai	retry, provider-retry, retry-profile-classifiers	133 passed (3 files)
packages/coding-agent	all 21 test/suite/retry-fallback*.test.ts	242 passed (21 files)
packages/coding-agent	5 stall suites (stream-start-stall-*, retry-fallback-stall-shared-budget, ...)	18 passed (5 files)

Stall handling (#1740) is verified intact by the last row: it is the silence class and keeps its own bounds, copy and shared retry budget.

One file-level failure, classified environmental, not a regression: packages/agent/test/agent-loop.test.ts fails to load with Cannot find package '@earendil-works/chord/context' imported from packages/agent/src/harness/context.ts. The same file fails identically on a BASE worktree of main (a7dee82bb1) in the same environment - the linked node_modules has chord unbuilt. It is a setup artifact of running vitest against a symlinked worktree, not something this diff touches; CI's own Test jobs are the authority.

Changelog gate: the PR carries no-changelog. The entry for #1739 was deleted from the released [2026.9.16-2] section (same treatment #1756 gave the #1754 entry) and no "Removed" note replaces it, because the decision is that the feature reads as never having shipped rather than as an added-then-removed behaviour.