docs(changes): describe the withdrawn rate guard without naming its symbols

[code-yeongyu]

Follow-up to #1761 (issue #1759).

The fork trackers that recorded the removal named the very identifiers they describe as deleted - stream-throughput-watchdog.ts, StreamRateMeter, formatWorkingRateSegment, minThroughputTokensPerSecond and the rest - so a grep of the tree still found the withdrawn feature in four changes.md files. That was the last trace.

Each entry now describes what left those files in prose ("the loop-config option that carried the rate thresholds", "the per-turn rate meter", "the three retry.provider rate fields") while keeping:

• every production file path the changes.md policy matches on, and
• all four canonical sections (What changed, Why, Why an extension could not handle it, Expected merge conflict zones).

Docs only: no production path is touched, so the changelog gate audits nothing (changelog-gate: PASS - changes.md coverage complete (0 production path(s) covered); no runtime source changes detected, run locally against this base).

After this lands, on main:

git grep -E 'minThroughputTokensPerSecond|StreamRateMeter|formatWorkingRateSegment|StreamThroughputDegraded|stream-throughput-watchdog'
# (no matches)

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Summary by cubic

Rewrites the four changes.md entries that recorded the withdrawn rate guard so they no longer name the deleted symbols. The earlier entries spelled out identifiers like stream-throughput-watchdog.ts and minThroughputTokensPerSecond, leaving the withdrawn feature findable via git grep; each entry now describes the removal in prose while keeping the file paths the changelog policy matches on.

• No production path is touched, so the changelog gate audits nothing.
• Each entry keeps all four canonical sections (What changed, Why, Why an extension could not handle it, Expected merge conflict zones).

^{Written for commit 2329a19. Summary will update on new commits.}