build: run lathe generator from the go.mod tool directive

[samzong]

What's changed?

• Add tool github.com/lathe-cli/lathe/cmd/lathe to go.mod at lathe v0.6.3 and run go tool lathe bootstrap in cli-sync instead of go run ...@<pinned-version>.
• Template default lathe_version is now v0.6.3; the replacement only touches go.mod because Makefile no longer carries a version.
• Regenerate internal/generated and skills/appctl with lathe v0.6.3.

Why

• The generator version was pinned in Makefile separately from the runtime in go.mod. Upgrading the runtime or replacing it with a local checkout left the generator on a different version, and the generated CLI failed to start. With the tool directive, one go.mod entry selects both.

Verification

• Starter check and the CI git diff --exit-code set after cli-sync: clean.
• lathe init --template <this branch> followed by the starter check passes with the v0.6.3 release binary, with LATHE_INIT_VERSION=v0.6.2 (generator and runtime both resolve to v0.6.2), and after go mod edit -replace github.com/lathe-cli/lathe=<local checkout> as in the Lathe release gate.