Support a local, gitignored coding-conventions override

[MickCrozier]

Summary

• Add agent/docs/CODING_CONVENTIONS.local.md as a gitignored override path so developers can maintain personal coding conventions without editing the checked-in team file.
• The agent's mandatory pre-write step (.claude/CLAUDE.md, symlinked as AGENTS.md) now checks for the local file first and follows it exclusively when present, falling back to the checked-in CODING_CONVENTIONS.md otherwise.
• Documented both customization paths (team-wide checked-in vs. personal local-only) in README.md and at the top of CODING_CONVENTIONS.md.

Test plan

• Confirm agent/docs/CODING_CONVENTIONS.local.md is correctly ignored by git (git status shows it untracked/ignored)
• Start a new agent session, create a local override with a distinct rule, and confirm generated FileMaker code follows the local file instead of the checked-in one

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