fix(agents): sharpen Skim agent usage discipline and heatmap scoping

[dean0x]

Summary

Five targeted edits to src/assets/agents/skim.md that sharpen the Skim agent's file-reading discipline, heatmap usage, and CLI invocation. No build step required — agent assets are the single source of truth.

Behavioral guidance was the gap: the cascade description (full → minimal → structure → signatures → types) was already correct per skim --help 2.11.0. These edits address what was missing.

Changes

Edit 1 — PATH-first invocation: Intro paragraph now instructs the agent to prefer the skim binary when on PATH (command -v skim) and fall back to npx rskim, so installations with the binary avoid the npm spawning overhead.

Edit 2 — Branch-scoped heatmap: Step 4 guidance adds --diff <base-branch> for feature-branch runs to scope findings to touched files only, plus --window sprint|month|quarter for recency tuning and --top N for result-count control.

Edit 3 — One-view-per-file discipline: Step 5 replaces the binary skim/Read split with a three-tier model: structure (default skim), logic (--mode pseudo), or exact content (Read tool). Includes explicit anti-pattern guidance: don't skim a file you already know you'll Read.

Edit 4 — Reference table and prose/config note: Two heatmap rows added (--diff and --window). A sentence below the table notes that skim handles .md/.json/.yaml/.toml files with heading/key structural views.

Edit 5 — Principle 2 reworded: From the binary "rskim for structure, Read for content" to the three-tier "One view per file" formulation that matches Step 5.

Verification

Empirically verified against skim --help 2.11.0: cascade (full → minimal → structure → signatures → types), --diff, --window, --top, and prose/config support are all present. The cascade description in the file was already correct — behavioral guidance was the only gap.

Test suite: all 30 test files passed, EXIT=0 (177 shell-hook tests, 136 proxy tests, 87 agent-frontmatter tests, and 22 other suites — no failures, no flakes on this run).