fix: align system prompt with PI rules

[5258MF]

Summary

This PR refreshes and replaces #5 on the current monorepo. It aligns the shared OpenCode system prompt with the more mature PI implementation by embedding the complete compression guidance exported by acp-kernel@0.0.19.

The change is prompt-only: it does not modify tool registration, V1/V2 message conversion, state, schemas, or kernel behavior.

What changed

• Embed these four kernel constants verbatim and exactly once:
  • COMPRESS_PHILOSOPHY
  • HOW_TO_COMPRESS_RULES
  • TIER2_DISTILL_RULES
  • TIER3_CONDENSE_RULES
• Expand the OpenCode-specific guide for:
  • bili_compress, including single, batch, and block-boundary examples
  • bili_decompress, including block and single-message refs, default output modes, inline:true, and full:true
  • bili_search, including search-before-decompress guidance
  • bili_status, including the supported scope and view modes
• Explain that compressed summaries are historical model-generated metadata, not current user instructions.
• Add host-neutral ACP-tag wording because the shared prompt is used by both host paths.
• Remove the inaccurate adaptive-threshold claim. In acp-kernel@0.0.19, the configured floor and cap are both 50,000 tokens.
• Add four focused regression tests.

Known limitation: compress vs bili_compress

acp-kernel@0.0.19 uses the generic tool name compress in HOW_TO_COMPRESS_RULES and in its injected nudge text, while this plugin registers the tool as bili_compress.

This PR deliberately:

• keeps all kernel constants verbatim;
• does not replace compress with bili_compress;
• does not add an alias;
• does not add a temporary mapping explanation to the prompt; and
• does not claim that the naming mismatch is fixed.

Before this change, the mismatch was exposed when the kernel injected a nudge. After this change, the same generic wording also appears in the persistent system prompt because HOW_TO_COMPRESS_RULES is now embedded there. The model can see the separately documented registered tool name, but it may still attempt to call the unavailable compress name.

A precise OpenCode adapter mapping, if adopted, should be reviewed in a separate follow-up PR after this prompt PR is merged.

Scope

• packages/core/src/system-prompt.ts
• packages/billion-context-opencode/tests/system-prompt.test.ts
• devlog/2026-08-13_system-prompt-align/

No dependency or lockfile changes are included.

Validation

• npm run typecheck — pass
• npm test — pass, 30/30 (4 new tests)
• npm run build — pass
• bash scripts/ci/check-pr.sh 2026-08-13_system-prompt-align upstream/master — pass
• git diff --check — pass

The rendered prompt is 15,261 characters (approximately 3,815 tokens using the rough characters/4 estimate; this is not an exact tokenizer count).

Independent review

Two independent review agents approved the final change. Their adversarial review led to two accuracy fixes before approval: protected tool output is excluded from compression but may still be emergency-truncated, and bili_decompress supports both block IDs and single-message refs.

Risk and rollback

The principal risk is the explicitly unresolved tool-name mismatch above, plus the larger static prompt. Rollback requires reverting only this prompt/test/devlog change; there is no persisted-data or schema migration.

[5258MF]

Dual-agent review record

Two independent review agents reviewed the source change and approved the final revision.

• Reviewer 1 checked the four verbatim kernel constants, actual tool schemas/capabilities, V1/V2-neutral wording, threshold claims, type safety, compatibility scope, tests, and devlog. Final result: APPROVE, no blocking findings.
• Reviewer 2 performed an adversarial review and initially found two accuracy issues: protected tool output was incorrectly described as always intact, and single-message decompression had been omitted. Both were corrected with focused positive/negative assertions. The reviewer then rechecked the final diff. Final result: APPROVE, no remaining blocking findings.

Both reviewers explicitly confirmed that this PR introduces no compress to bili_compress conversion or alias. The unresolved naming mismatch remains a disclosed, accepted non-blocking risk for this PR.

Final local verification:

• npm run typecheck: pass
• npm test: 30/30 pass
• npm run build: pass
• PR validation script: pass
• git diff --check: pass