feat(mecated): log version on startup

[danbarr]

Summary

• Emit mecated's build identifier in a structured startup INFO log.
• Document the startup version field in the standalone-server deployment guide.

Development stage

• Spike / Routine — acceptance-plan spine exempt; Spike evidence does not ship as-is

Contract linkage

• Work classification: Routine
• Classification rationale: A localized operational-log and documentation improvement with no API, configuration, persistence, or trust-boundary change.
• Decision record: None — no durable architectural decision required
• Human waiver of spine: No
• Acceptance plan: N/A — Routine work
• Human decisions resolved and recorded: N/A — Routine work
• Plan / Interface PR: N/A
• Approved commit baseline: N/A
• Combined/exemption rationale: Routine work

Interface conformance

• No public interface change. At startup, mecated logs msg="mecated starting" version=<build-id>.

Issue relationship

No issue provided.

Type of change

• New feature
• Documentation/process

Test plan

Baseline checks

• Linting (task lint)
• Offline test suite (task test) — an unrelated existing test timed out: internal/adapter/server.TestSDKTypescriptRelease_Scenario11_TagVersionParity exceeded its 10-minute timeout.
• Offline demo (go run ./cmd/mecademo)
• Markdown changed: docs generation/link checks (task docs)
• User docs/user-facing behavior changed: site build (task site:build)
• Guarded engine API affected: compatibility check (task api:check) — included in task test before the unrelated timeout

Changes

File	Change
cmd/mecated/main.go	Log the build identifier after configuring the daemon logger.
user-docs/building/deployment/mecated.md	Document the startup version log field.

User-facing change

Every mecated serve and mecated acp startup now includes the build identifier in the configured log output, making operational logs attributable to the exact binary version.

Special notes for reviewers

The version uses the existing internal/buildinfo.BuildID, which is linker-stamped in release builds and has the established VCS/dev fallback for source builds.