refactor: delegate historical-image strip to acp-kernel wire primitive (#617)

[ranxianglei]

What

Completes the rewire step planned in #617: the opt-in historical-image strip that #618 implemented locally (src/strip-images.ts) now delegates to the kernel's wire-layer primitive.

• src/server.ts: stripHistoricalImages imported from acp-kernel/wire alongside the protocol codecs; the runPrepare injection point is untouched (same call shape, same [debug] strip-images log line, same identity-no-op semantics).
• src/compress-settings.ts: DEFAULT_STRIP_IMAGES_KEEP_RECENT = 5 moved here — host-side policy stays host-side, mechanism lives in the kernel.
• src/config.ts: stripImages docstring updated to reference the kernel primitive (was pointing at the deleted local module).
• src/strip-images.ts: deleted (per-protocol traversal duplicated from src/image-tokens.ts).
• tests/strip-images.test.ts: retargeted at the bundled kernel export — retained as host-level regression coverage of the exact code path the proxy runs.
• No pin bump: already pinned acp-kernel@0.0.59, which ships the primitive (kernel fix(responses): remap round-2 message lifecycle so codex stops dropping text deltas #215, released in v0.0.58+).

Why

Single source of truth: the pi adapter (billion-context-pi#321, PR billion-context-pi#324) shares the kernel implementation, so cutoff / [image]-placeholder / no-op semantics can't drift between hosts. Siblings: acp-kernel#215 (mechanism), acp-kernel#218 (v0.0.58 release).

Validation (against published acp-kernel@0.0.59, clean npm ci)

• tsc --noEmit --project tsconfig.build.json — clean
• full suite: 1240 tests, 1239 pass, 0 regressions. The one failure (resolveClientCommand: codex/claude resolve to themselves) fails identically on pristine master in this sandbox — /usr/bin/codex + /usr/bin/claude are installed here, CI runners have neither; pre-existing, environment-dependent, unrelated to this change.
• strip suites (tests/strip-images.test.ts + tests/strip-images-compact.test.ts): 9/9 pass against the kernel export
• tsup build succeeds; acp-kernel/wire inlined into dist/index.js as before

No behavior change: identical semantics (recent-N kept, image-only messages collapse to an [image] placeholder, identity no-op when nothing changed), verified by the retargeted tests running against the kernel implementation.

[github-actions]

📦 Built Package Artifact

Branch: 2026-09-08_issue617-strip-images-kernel-adopt (bd4fecc)

Option A — Install from npm PR tag (recommended)

npm install -g billion-context@pr-635

Each push to this PR publishes a new version under the pr-635 npm tag.

Option B — Download artifact

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

tar xzf billion-context-pr635.tgz
npm install -g package

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