perf(agent-package): index archive entries by skill path

[Yevanchen]

Summary

• Index non-empty package archive entries by declared skill path in one pass.
• Preserve manifest order, archive-entry order, missing-skill behavior, and overlapping skill-root projection.

Why

• Package import previously filtered every archive entry once per declared skill, for O(skills × entries) matching work.
• The new prefix index reduces matching to O(skills + entries × path depth + matched assets).
• A synthetic 300-skill, 6,000-entry archive improved from 41.83 ms to 0.86 ms per import (about 49×) on the same runner.

Closes YEF-1078

Verification

• Commands:
  • just test-file pkgs/agent-package/tests/archive-entry-admission.test.ts
  • just test-package @mosoo/agent-package
  • just tc-package @mosoo/agent-package
  • bun run --filter @mosoo/agent-package lint
  • just fmt-check-path pkgs/agent-package/src/archive-assets.ts
  • just fmt-check-path pkgs/agent-package/tests/archive-entry-admission.test.ts
  • just commit-check
  • just check (format, docs, lint, and typecheck passed; test phase stopped on 11 unmodified Driver process-watchdog/PID timing failures on macOS)
• Manual steps: N/A
• Not run: N/A

Impact

• User/API/contract changes: None; package import output and issue behavior are preserved.
• Generated files / GraphQL / DB / lockfile: None.
• Env or config changes: None.
• Risk and rollback: Low; rollback by reverting the single commit.

Review

• Closest review areas: archive path-prefix matching and overlapping declared skill roots.
• Known trade-offs: The index adds memory proportional to declared skill paths and matched assets, while avoiding repeated full archive scans.