chore: untrack local specification records

[Daxiongmao87]

Summary

• Ignore the repository-standard local spec/ development-record directory.
• Retain the existing legacy specs/ ignore rule and E2E exceptions.
• Remove the completed Discord announcement specification from Git tracking while leaving local specification records available to development tooling.

Cause

The ignore file covered plural specs/, but the development policy and existing record use singular spec/. Commit c98c090 therefore added spec/001-announce-new-issues-to-discord.md as a tracked file because no matching ignore rule existed. Adding an ignore rule alone would not untrack an already indexed file, so this change also removes that file from the index.

Empirical Verification (UTRs)

This PR changes only .gitignore and a development-only Markdown record; it does not change runtime code.

$ git diff --name-status origin/main...HEAD
M       .gitignore
D       spec/001-announce-new-issues-to-discord.md

$ git ls-files --stage -- spec/001-announce-new-issues-to-discord.md
(no output; the file is no longer tracked)

$ git check-ignore -v --no-index spec/001-announce-new-issues-to-discord.md
.gitignore:8:spec/ spec/001-announce-new-issues-to-discord.md