fix(reasoning): close rounds on tool-result evidence, not user messages

[ranxianglei]

Fixes #348

Problem

The compress.reasoning drop (shipped in #339, aligned with opencode-acp #377) closed a turn only when a genuine user message existed after the compress call. Long agentic sessions have no user messages after the opening prompt (observed: session 01a086ff — 64 entries, only 2 genuine user messages, 30 compress calls), so every compress round stayed "active" forever and its thinking survived as a permanent uncompressible floor. Observed with threshold: 1: 0 drop events; retained thinking 20612 / 8411 / 10581 chars.

Fix

Round closure is now judged on round evidence:

• every compress toolCall in the message has its toolResult (role toolResult, matching toolCallId) at a later index, and
• at least one message exists after that result (the round has demonstrably moved on).

Safety gates preserved:

• in-flight calls (result missing, or result still the last message) are never touched;
• the nudge push runs after the drop, so a synthetic user-role nudge can never close a round;
• per-provider compress.providers.<name>.reasoning.drop = false remains the escape hatch for reasoning-replay providers (GLM).

Verification

• Full pipeline repro on the real session 01a086ff (threshold: 1): compress-anchor thinking 20612 → 0 chars, deterministic across runs; non-compress thinking (82–1893 chars) untouched.
• Test suite rewritten + extended: 653 pass / 0 fail (3 pre-existing skips). New reasoning-drop never fires in single-user-message marathon sessions (closed-turn gate unreachable) #348 scenarios: no-user-message assistant chain closes, result pending, result-before-call, mismatched id, mixed compress+other calls.
• npm run typecheck clean.

Repro artifacts: ~/tmp/repro-rd.mjs (drives the dist transform with the real session entries).

[github-actions]

📦 Built Extension Artifact

Branch: 2026-09-10_reasoning-round-evidence (bc99b8c)

Option A — Install from npm PR tag (recommended)

pi install npm:billion-context-pi@pr-349

Each push to this PR publishes a new version under the pr-349 npm tag.

Option B — Download artifact

1. Download the artifact from the Actions run
2. Extract the tarball and install:

tar xzf billion-context-pi-pr349.tgz
pi install ./package

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