feat(api): scaffold packages/api with health group contract (Step 1a)

[guidefari]

Step 1a of the Hono -> Effect HttpApi migration (docs/migration-effect-http-api.md), redone after closing #136 for scope creep. This PR is contract-only, on purpose.

What

• New leaf workspace package packages/api (depends only on effect@4.0.0-beta.93) so server-only code cannot leak into the www client bundle.
• Health group contract: live (/health/live), ready (/health/ready), check (/health) endpoints with response schemas mirroring current Hono behavior.
• ReadinessCheckFailedError (Schema.TaggedErrorClass, httpApiStatus: 500) for the readiness failure path.
• Also includes the revised migration doc: the step breakdown is now split into smaller single-purpose check-ins (1a/1b, 2a/2b/2c, 3a/3b, ...) instead of the old coarse steps that let feat(api): scaffold @gbfm/api with Effect HttpApi health contract #136 balloon.

What this deliberately does NOT do

• No apps/vps changes. Nothing serves this contract yet.
• No unit tests yet -- that's 1b, kept separate so this PR is pure "does the contract typecheck and read well."
• No hand-rolled test scripts. Test policy in the revised doc: vitest only.

Verification

• bun --filter='@gbfm/api' typecheck -- clean
• bun run lint / bun run format:check -- clean

Next

1b: contract unit tests (schema decode/encode, error tagging) in packages/api.