fix(daemon): restore claim slow-log payload observability without gzip

[Hzzzzzx]

What

#4307 reverted "gzip daemon claim responses" (#4259). That revert was the right call for the gzip middleware, but it also removed — as collateral — the claim slow-log observability fields, leaving the /api/daemon/tasks/claim response-size signal blind in prod.

This PR restores only the observability half, with the gzip middleware deliberately left out.

Changes

• handler.go — re-add writeMeasuredJSON: same output as writeJSON but returns the encoded body size. It measures the uncompressed JSON length and is unrelated to transport compression.
• daemon.go (ClaimTaskByRuntime / logClaimEndpointSlow) — restore payload_bytes, agent_skill_count, builtin_skill_count, skill_payload_bytes on the slow-claim log, captured at the response-build sites.

Explicitly NOT included

• ❌ daemonJSONCompressor / chimw.Compress gzip middleware — the part #4307 actually targeted.
• ❌ No changes to router.go, no middleware, no route wiring.
• ❌ No structural refactor.

So this is a pure logging/diagnostics restore with no wire-behavior change.

Tests

• New TestWriteMeasuredJSONByteIdenticalToWriteJSON — table-driven, asserts writeMeasuredJSON and writeJSON emit byte-identical response bodies (and matching length / status / Content-Type) across many inputs: HTML-escapable chars, unicode, nested structures, and a large claim payload with skills. This locks the "no wire change" invariant so it is provable and regression-guarded.
• Restored TestLogClaimEndpointSlowIncludesPayloadFields — asserts the four fields reach the log line.

Verification

• gofmt -l — clean
• go vet ./internal/handler/ — clean
• go test ./internal/handler/ — ok (full package, ~7s)
• go build ./... — ok

🤖 Generated with Claude Code

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