feat: auto-ignore .codemap/ when setting up codemap in a repo

[JordanCoin]

Problem

codemap writes all its runtime state to .codemap/ (config, event log, watch PID, handoffs, hub cache) — but it never tells git to ignore that directory. This repo's own .gitignore has .codemap/ only because it was added by hand. Every other project a user runs codemap in gets an unprotected .codemap/ that they'll accidentally commit.

This came out of a discussion about how codemap should behave across git worktrees: because .codemap/ is (correctly) per-working-directory runtime state, the tool should own making sure its own footprint is ignored — per git root, across worktrees and nested repos.

What this does

New ensureCodemapIgnored(root, mode) helper:

• No-op if git already ignores .codemap/ — checked via git check-ignore, so it honors global excludes, existing .gitignore rules, and negations. No duplicate lines.
• Two tiers:
  • ignoreTracked → writes .codemap/ to the repo's tracked .gitignore (team convention; every clone/worktree inherits it).
  • ignoreLocal → writes to <git-common-dir>/info/exclude (local-only, no tracked diff, shared across worktrees). Wired in a follow-up.
• Safe: silent no-op outside a git work tree; creates the ignore file if missing; preserves existing content + trailing newline.

Wired the tracked tier into config init and setup through a shared reportEnsureIgnored seam that prints a one-line Added .codemap/ to .gitignore notice.

Testing (TDD)

Tests written first, red → green. go test ./... fully green, go vet clean.

• TestEnsureCodemapIgnored — tracked write, no-op-when-ignored, create-when-missing, append-preserving-newline, ignoreLocal→info/exclude, non-git skip.
• TestReportEnsureIgnored — notice on first run, silent when already ignored, silent outside git.

Also smoke-tested the built binary: codemap config init in a fresh repo prints the notice, git check-ignore confirms the rule, and a second run stays idempotent.

Follow-ups (not in this PR)

• Wire the ignoreLocal safety-net into the lazy .codemap/ creation paths (cmd/hooks.go, watch/daemon.go) — needs the helper promoted to a shared package since watch can't call cmd's unexported helper.
• Multi-repo: ensure each git root codemap materializes state under gets its own rule.

🤖 Generated with Claude Code

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.68421% with 20 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/gitignore.go	72.97%	11 Missing and 9 partials ⚠️

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