feat(fastdeploy): treat blocks.gen.json as content in the content-only gate

[hugo-ccabral]

Studio content commits touch two paths, not one: the decofile blocks (.deco/blocks/**) and the regenerated bundled snapshot src/server/cms/blocks.gen.json (kept in lockstep for HMR). The content-only gate only accepted the former — so every real Studio commit was classified as a code change and never fast-deployed.

Content-only now means: every changed file is under .deco/blocks/ or equals src/server/cms/blocks.gen.json.

• internal/deploy/target.go — extracted isContentPath (prefix or exact-file match); blocksGenFile constant documents why the gen file carries no code change (it's derived from the blocks).
• internal/deploy/target_test.go — new unit tests for isContentOnly (incl. the real Studio-commit shape and the .deco/blocks-old sibling case) and decofileName (sanitization + 63-char cap).
• docs/fast-deploy-webhook.md — updated rule description.

Note: the KV sync itself is unchanged — the syncer reads .deco/blocks/*.json; the gen file just rides along in the commit.

🤖 Generated with Claude Code

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Summary by cubic

Expand the content-only fast-deploy gate to include the generated snapshot src/server/cms/blocks.gen.json, so Studio content commits fast-deploy instead of triggering a full build. KV sync behavior is unchanged.

• Bug Fixes
  • Content-only now accepts changes under .deco/blocks/** or exactly src/server/cms/blocks.gen.json (extracted isContentPath, added blocksGenFile).
  • Added tests for isContentOnly and decofileName (covers Studio commit shape and edge cases).
  • Updated docs/fast-deploy-webhook.md to reflect the new rule.

^{Written for commit 8274e2b. Summary will update on new commits.}