release: v0.4.0 (cleanups + Nexus broker stable release)

[tbitcs]

Closes the four optional cleanups (#4-#7 from the project-done checklist) and bumps the package to 0.4.0 ready for a stable release.

REQs / TESTs added

• REQ-104 / TEST-104 - scripts/sync_workitems.py keeps .specsmith/workitems.json mirrored to the implemented REQ/TEST set; all 107 WORK items are now status=complete.
• REQ-105 / TEST-105 - .specsmith/runs/WI-NEXUS-011/logs.txt refreshed with a fresh nexus_smoke.py probe, UTC timestamp, host docker + GPU info, and the hardware reason the live container can not be reached on this workstation (12 GB VRAM vs ~20 GB needed by the 32B GPTQ-Int8 model).
• REQ-106 / TEST-106 - VS Code extension parity: specsmith.runPreflight, specsmith.runVerify, specsmith.toggleWhy shipped in BitConcepts/specsmith-vscode#28 (already merged).
• REQ-107 / TEST-107 - ARCHITECTURE.md gained a 'Current State (post-WI-NEXUS-023, target release 0.4.0)' section that lists the realized broker, harness, retry strategies, CI baseline, VS Code parity, live-smoke evidence note, and documentation surface.

Release prep

• pyproject.toml: 0.3.13 -> 0.4.0.
• CHANGELOG.md: promoted [Unreleased] to [0.4.0] - 2026-04-28 with the full feature inventory for REQ-084..REQ-107; added the version-link footer.
• Governance state synced: 107 reqs / 107 test cases / 107 WORK items / 5 chained ledger entries (WI-NEXUS-024..028).

Verification

pytest:                  259 passed, 1 skipped
ruff check:              All checks passed!
ruff format --check:     112 files already formatted
mypy src/specsmith/:     Success: no issues found in 69 source files
sync_governance_state:   Synced 107 requirements and 107 test cases.
sync_workitems:          Synced 107 work items (107 complete, 0 pending)