feat(compact): route /compact through the ACP compression pipeline

[ranxianglei]

What

Intercept Pi's session_before_compact and route it through the ACP compression pipeline instead of cancelling native compaction: pick a compressible span, summarize it with a model, applyCompression, then hand the summary back to Pi as the compaction result.

How

1. processTurn → get compressible ranges
2. selectRangeSpan → pick a span ≥ minCompressRange (seeds on largest range, expands by smallest gap)
3. summarizeRange → call a model (compressModel or session model) with the slice
4. applyCompression → create an ACP block
5. return { compaction: { summary, firstKeptEntryId, tokensBefore } } so Pi stores it

Fallback: on any failure (no compressible ranges, no usable model, unparseable response, kernel rejection, exception) return undefined → Pi falls back to its own compaction. Context is never lost.

Summary prompt uses the kernel Prompts (key design choice)

The summary-generation system prompt is built from the kernel's load-bearing compression rules (compressPhilosophy + howToCompressRules) via the Prompts interface — not a hand-rolled constant. This means:

• /compact honors acp.json prompt overrides (the same Prompts the compress tool + tier-1 compression use)
• stays consistent with the rest of the ACP pipeline (no drift between /compact and the compress tool)
• tier-1 howToCompressRules are the right rule set (/compact compresses an old contiguous range, not a tier-2/3 distillation)

Config

compressModel in ~/.pi/acp.json as provider:modelId (e.g. "openai:gpt-4o"). When unset, falls back to the current session model — so /compact works with zero config.

Dependency

Adds @earendil-works/pi-ai as a devDependency for the complete() type (kept external at runtime via tsup — resolves inside Pi where pi-ai is installed). ctx.modelRegistry is a synchronous facade that deliberately does not expose complete()/stream(), so the standalone import is the supported path.

Files

• src/auto-compress.ts (new) — range helpers + summarizeRange + buildSummaryPrompt
• src/index.ts — wireCompactionDisable interception
• tests/auto-compress.test.ts (new) — 19 tests
• tests/integration.test.ts — updated compaction-contract test

Validation

typecheck ✓ / 294 tests pass (19 new) ✓ / build ✓

Not included (split into separate PRs)

• usageTriggerPercent auto-nudge (dropped — redundant with kernel growth/overLimit triggers)
• acp_summary_ surfaced as inline user message (dropped — redundant with the block; compress tool is the compression)
• capToolOutput refactor (separate concern)
• i18n, clean-output, symlink fix → feat(i18n): localize user-facing slash-command output (display-only) #142 / feat(guardrails): clean noisy bash output (dedupe + denoise) #141 / fix(decompress): harden toFile against symlink path-escape #140

Extracted from #119 (thanks @21307369).