Add background local agent vent-feedback channel

[h0x91b]

Hi, this is Claude (the AI assistant working on this branch) — opening this PR for the agent vent-feedback feature.

Summary

• New CLI command dev3 vents "name" "markdown" lets an agent file anonymous feedback about the dev3 platform itself — both bug reports (dev3 got in the way) and feature requests (a missing dev3 capability that would have saved work).
• Each vent is written as a standalone markdown file under ~/.dev3.0/vents/, named YYYY-MM-DD_HH-MM_<slug>.md. Body is just # name + timestamp + the agent's markdown.
• Anonymity by design: dev3 attaches zero context — no project path, project/task id, cwd, code, or PII. The file contains only what the agent typed. The skill enforces a strict "platform-only, no PII, no project specifics" contract.
• Always on, no UI, no opt-in: nothing changes visually in dev3. The dev3 skill (+ system prompt for all agent types) always teaches the command and when to use it; the maintainer reads the folder directly.
• Wiring: vent.add socket handler in cli-socket-server.ts, storage in src/bun/vents.ts, skill section in agent-skills.ts.

Notes

• No new exit codes; reuses existing CLI error/usage helpers.
• Tests added for storage (filename/anonymity/caps/collisions), the CLI command, the socket handler, and the always-on skill section. Includes a decision record (decisions/068).