fix(launcher): ensure configured data root before verify mount; docs for M1 (#4)

- verify.sh now calls bootstrap_runtime_dir after load_config; previously a
  reset-then-verify flow let Docker auto-create a root-owned mount source
- common.sh: fail with clear guidance when data root exists but is not writable
- README: configuration section, bootstrap usage, selftest entry point
- BUILD-LOG: Phase 5 entries with corrections

E2E (clean slate): 20/20 selftests; bootstrap idempotent; config-driven
hello/verify MOSAIC_HELLO_OK; negative marker exit 1; reset + rerun green;
config checksum unchanged across the entire flow.

Closes #4

BUILD-LOG.md

@@ -106,4 +106,35 @@ No credentials are recorded in this file.
 
 All 11 acceptance criteria demonstrated. The real model request passed.
 
+---
+
+## Phase 5: Configuration-driven Hello World (M1)
+
+### Entry 5.1 — before
+
+- Timestamp: 2026-09-03
+- Intended action: Make the container POC configuration-driven. Baseline committed and tagged `poc-container-hello-v0`. Milestone M1 tracked in Gitea (issues #1-#4): (T1) config module with idempotent bootstrap and strict v1 validation; (T2) wire scripts and compose to config.json with fail-closed behavior; (T3) sandboxed config selftests; (T4) E2E verification and documentation.
+- Reason: Per docs/plans/2026-09-02_atomic-mosaic-foundation.md — config.json must be the sole discovery entry point; updates and runs must never corrupt or invent configuration.
+- Expected result: All M1 acceptance criteria pass; Hello World reproducible from configuration alone.
+
+### Entry 5.2 — after
+
+- Timestamp: 2026-09-03
+- Commands run: `scripts/test-config.sh` (20 cases); fail-closed checks (compose without launcher env, verify/reset with missing config); `scripts/bootstrap.sh`; config-driven `scripts/hello.sh`, `scripts/verify.sh`, negative marker test, sandboxed reset symlink refusal (canary survived), real reset + bootstrap + build + verify; config checksum comparison across the entire flow.
+- Observed result:
+  - Config selftests: 20 passed, 0 failed.
+  - Fail-closed confirmed: compose exits 1 without launcher env; verify/reset exit 1 on missing config before any mutation.
+  - Bootstrap created `~/.config/mosaic-dev/config.json` exclusively; second run validated without rewriting (content + mtime unchanged).
+  - Config-driven hello/verify returned exactly `MOSAIC_HELLO_OK`; verify exit 0; negative marker test exit 1.
+  - Reset refused symlinked dataRoot; canary file survived; real reset removed only the configured data root.
+  - config.json checksum unchanged across hello/verify/reset/bootstrap/build/verify.
+- Failure or correction:
+  1. Selftest harness bug: `cfg` helper invoked without a body for the symlink case (`$2: unbound variable`). Fixed in the harness; product code unaffected.
+  2. E2E rerun-after-reset failure: `verify.sh` did not ensure the configured data root existed before the container mount. With the data root absent, Docker auto-created the host path as root:root, and the container's uid-1000 user could not write the generated system prompt. Fixed by calling `bootstrap_runtime_dir` in `verify.sh`; also hardened it to fail with a clear message when the data root exists but is not writable (root-owned leftover). Clean-slate E2E rerun: all steps green.
+- Credential check: no credential material in config, scripts, logs, or test output.
+
+## Result (M1)
+
+Configuration-driven Hello World verified. `main` merged with M1 and tagged `config-hello-v1`.
+