docs(adapters): adapter seam docs + recorded M4 E2E, release 0.0.5 (#19)

- README: Runtime adapters section (contract summary, selection, mission
  injection point); BUILD-LOG Phase 8 entries
- E2E: 24+24+14 selftests green; verify PASS; 0.0.5 packaged and
  health-gated activated; mission-bearing fixture task succeeded through
  the real pi adapter; config checksum unchanged

Closes #19

BUILD-LOG.md

@@ -192,4 +192,33 @@ Mission/task layer verified end-to-end. `main` merged with M2 and tagged `missio
 
 Release model and safe updates verified by drills. `main` merged with M3 and tagged `release-model-v1`.
 
+---
+
+## Phase 8: Runtime adapter seam (M4)
+
+### Entry 8.1 — before
+
+- Timestamp: 2026-09-03
+- Intended action: Formalize the harness boundary (Gitea milestone M4, issues #16-#19): documented adapter contract under /opt/mosaic/adapters/<name>/adapter.sh; run-agent.sh becomes a dispatcher; pi extracted unchanged; deterministic mock adapter for provider-free seam tests; config gains optional execution.adapter (default pi, configVersion unchanged); mission directives gain their sanctioned injection point via the run snapshot; RELEASE bumps to 0.0.5 with a health-gated activation.
+- Reason: Future harnesses (Claude, Codex, OpenCode) must be additive — one directory each — and mission content needs a single sanctioned path into the runtime.
+- Expected result: All suites green including new deterministic seam cases; 0.0.5 activated by health gate; mission-bearing run recorded.
+
+### Entry 8.2 — after
+
+- Timestamp: 2026-09-03
+- Commands run: scripts/test-config.sh; scripts/test-task.sh; scripts/test-release.sh; manual seam drills (mock verbatim, unknown/traversal adapter refusal); mission injection checks; release package + activate for 0.0.5.
+- Observed result:
+  - Config suite 24/24 (adapter default/validation/env export).
+  - Task suite 24/24 including deterministic mock cases (gate pass, expect-mismatch with reason, unknown adapter fail-closed) and mission injection asserted by prompt content.
+  - Release suite 14/14; image mosaic-poc-agent:0.84.4-r0.0.5 packaged and activated via exact-marker health gate.
+  - Mission directives now flow: task -> run snapshot -> container env -> generated prompt MISSION (runtime) section.
+- Failure or correction:
+  1. Selection authority settled: load_config always exports MOSAIC_ADAPTER from config; environment overrides for scripts are therefore not a supported selection path (by design).
+  2. Selftest harness: three authoring defects fixed (helpers used before definition; one config file reused across cases leaking adapter state; a static mission fixture asserted against distinctive seam directives; plus an accidentally duplicated live block removed).
+- Credential check: no credential material in adapters, prompts, run records, or logs.
+
+## Result (M4)
+
+Adapter seam verified; harness boundary is now additive by construction. `main` merged with M4 and tagged `adapter-seam-v1`.
+