[codex] Prepare briefprint 0.4.0 release hygiene

[dd3ok]

Summary

• bump Briefprint metadata/runtime version to 0.4.0
• expand the SKILL.md description to the 320-character repo-local routing cap with clearer positive and negative boundaries
• add a 0.4.0 changelog entry covering the recent release hygiene and skill-bundle changes

Why

The prior 260-character description cap forced terse wording and left no room for routing clarity. The recent briefprint rebrand, installable bundle path, cache explain output, incident lifecycle demo, sensitive alias, secret redaction, Korean README, and metadata cleanup are better represented as a minor 0.4.0 release than a patch-only update.

Impact

SKILL_VERSION is part of the document-summary cache key. After upgrading to 0.4.0, prior 0.3.x document-summary cache entries are treated as misses at the release boundary.

Validation

• PYTHONPATH=src python -m pytest -q -> 164 passed, 2 skipped
• PYTHONPATH=src python scripts/validate_skill.py -> OK
• git diff --check origin/main...HEAD -> clean
• README benchmark rerun matched the documented numbers: 7 scenarios, 1,748 naive tokens, 309 cache-aware tokens, 82.32% estimated savings, 16 hits, 4 misses

[gemini-code-assist]

Code Review

This pull request bumps the version of the document-briefing-cache package and the briefprint skill from 0.3.1 to 0.4.0 across all configuration files, source code, and tests. It also updates the skill description in SKILL.md to be more descriptive, adjusts the maximum description length constraint in the tests from 260 to 320 characters, and documents the changes in the changelog. There are no review comments, so I have no feedback to provide.

Important

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[dd3ok]

Self-review summary:

• Critical: none found.
• Important: none found after adding the changelog note that SKILL_VERSION participates in document-summary cache keys.
• Minor: none blocking.

Checked scope: PR only includes the changelog, 0.4.0 version sync, SKILL.md description/cap update, matching validator/test expectations, and mirrored agent metadata. README benchmark numbers were re-run locally and still match the documented values.

Validation reviewed:

• PYTHONPATH=src python -m pytest -q -> 164 passed, 2 skipped
• PYTHONPATH=src python scripts/validate_skill.py -> OK
• GitHub CI -> Python 3.10, 3.11, 3.12, and Distribution smoke passed

Remaining release work is post-merge only: create the v0.4.0 tag / GitHub release if this PR is merged.