feat(absorb): instant tool-result compression for small contexts (#14)

[ranxianglei]

Implements #14 (adapter side; kernel side is ranxianglei/acp-kernel#139).

What

Small-context setups (e.g. a 10K window) can't wait for the regular nudge-based compression — the model runs out of room. This adds the absorb tool: when enabled, large tool results (default ≥1000 estimated tokens) get a forced [ACP absorb] suffix demanding the model immediately call absorb({ ref, summary }). After absorption the original tool-call+result pair is hidden from later turns; the summary-carrying absorb call is the durable record. Absorb calls stay ordinary tool calls — the regular compression system can fold them into blocks later (orthogonal).

Config (acp.json)

{ "absorb": true }

or tuned: absorb.minToolTokens (1000), absorb.contextThresholdPct (0 = size alone decides), absorb.excludeTools ([]), absorb.toolName ("absorb"). Documented in CONFIGURATION.md + CONFIGURATION.zh-CN.md.

Changes

• src/absorb-tool.ts — tool handler (kernel applyAbsorb + parseAbsorbInput), follows compress-tool patterns (throw on error so pi marks isError)
• src/config.ts — AbsorbSettings, resolveAbsorb (boolean shorthand + % parsing), resolveConfig mapping
• src/index.ts — registration when enabled, re-registration on session_start (acp.json reload), absorb system-prompt section
• src/user-config.ts — absorb key (boolean or object)
• tests/absorb-tool.test.ts — 6 tests: config mapping, gated registration, forced prompt on large results only, hide-pair+keep-summary end-to-end, error paths, system prompt section
• CONFIGURATION.md / CONFIGURATION.zh-CN.md / CHANGELOG.md

Notes

• Merge AFTER acp-kernel#139 ships to npm, then the release commit bumps the acp-kernel pin (currently 0.0.41) per AGENTS.md §5. Until then CI may fail on the not-yet-released kernel APIs.
• Validated locally against the kernel branch build overlaid onto node_modules (AGENTS.md §5 pre-validation): 415/415 tests, typecheck clean, build clean.