fix(installer): trim codegraph skill description under Codex's 1024-char limit

[sunerpy]

Summary

The codegraph agent skill was invisible in OpenAI Codex's skill list (no warning), while sibling skills in the same ~/.agents/skills/ dir loaded fine.

Root cause, verified against Codex source openai/codex @ codex-rs/core-skills/src/loader.rs: the loader enforces MAX_DESCRIPTION_LEN = 1024 chars on a skill's front-matter description (after sanitize_single_line collapses whitespace to single spaces). An over-limit description becomes a SkillParseError::InvalidField, which routes the skill into outcome.errors (NOT outcome.skills) — because it's an invalid-field error rather than a context-budget omission, no warning is surfaced; the skill just silently vanishes.

The codegraph SKILL.md description folded to 1026 chars — 2 over the limit.

Changes

• skills/codegraph/SKILL.md: trim the front-matter description from 1026 → 706 folded chars, preserving the core trigger intent and a representative CJK trigger subset. Body (Part A / Part B) is byte-for-byte unchanged.
• crates/codegraph-cli/src/installer/skill.rs: add a fail-closed unit test skill_description_within_codex_limit that isolates the front-matter, requires exactly one non-empty description: > folded scalar, replicates Codex's fold (split_whitespace().join(" ")), and asserts the char count stays <= 1024 (Codex's contract) and <= 950 (maintenance margin). Std-only, no new dependency.

The install path (~/.agents/skills) is unchanged and correct — it is Codex's current user-scope skills root (~/.codex/skills is the deprecated back-compat location per the loader source). Only the over-long description was at fault.

Not touched: codex.rs skill_dir(), golden/equivalence fixtures, no hardcoded hashes.

Verification

• make ci green locally (fmt-check + clippy + test + guardrail)
• new guard test passes: installer::skill::tests::skill_description_within_codex_limit ... ok
• pre-push hook passed

[codecov]

Codecov Report

❌ Patch coverage is 84.31373% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/codegraph-cli/src/installer/skill.rs	84.31%	8 Missing ⚠️

❌ Your patch check has failed because the patch coverage (84.31%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #163      +/-   ##
==========================================
- Coverage   95.27%   95.26%   -0.02%     
==========================================
  Files         124      124              
  Lines       61140    61191      +51     
==========================================
+ Hits        58252    58293      +41     
- Misses       2888     2898      +10     

Files with missing lines	Coverage Δ
crates/codegraph-cli/src/installer/skill.rs	94.16% <84.31%> (-1.47%)	⬇️

... and 1 file with indirect coverage changes

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