fix(init): merge .gitignore with user content instead of overwriting (#6)

[kevinkod]

Closes #6.

Summary

Brownfield projects (Vite, Next, Astro, etc.) all ship a .gitignore. Before this change, specflow init --here reported it as a conflict and --force then backed up the user file and wrote specflow's stub — silently losing all framework ignores.

The fix introduces a new mergeable-project-root bundle category and an append-block strategy: specflow's .gitignore is wrapped in fence markers and merged into any existing user file, non-destructively, idempotently, and without --force.

Before / After

 # Logs
 logs
 *.log
 ...
 node_modules
 dist
 .DS_Store
 .vscode/*
+
+# --- Specflow: gitignore ---
+*.specflow.bak
+.specflow/config.yml.local
+# --- End Specflow: gitignore ---

User file fully preserved. No .gitignore.specflow.bak. No --force required.

Architectural shape

• New CoreCategory value mergeable-project-root (currently only .gitignore uses it).
• New TemplateFile.mergeBlock?: string field carrying the fence label.
• DenoFsWriter.detectConflicts skips entries with mergeBlock set.
• DenoFsWriter.writeBundle reads any pre-existing file and merges via mergeIntoFile() from the new src/domain/merge_block.ts helper.
• Lock SHA for mergeable files stores the canonical block body (trimmed) so it stays comparable against the body extracted from disk on upgrade reads.
• All 8 harnesses route mergeable-project-root to the same destination as project-root and stamp mergeBlock: \"gitignore\". No harness-specific divergence.

Test plan

• deno task test — 316 / 316 pass (was 313 before; +3 new brownfield tests).
• New integration tests in tests/integration/init_brownfield_gitignore_test.ts:
  • Brownfield: pre-existing Vite-style .gitignore → init succeeds without --force, both user lines and specflow block present, no .bak file created.
  • Idempotency: running init twice with --force does not duplicate the specflow block.
  • Greenfield: fresh init writes the .gitignore wrapped in fence markers (same shape regardless of whether merge was needed).
• End-to-end smoke against a real npm create vite@latest project — Vite's 24-line .gitignore (logs, node_modules, dist, .DS_Store, .vscode, etc.) preserved verbatim with the specflow block appended.

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