AI Implementation feature(1084): Blog Repository Clone and Pull Workflow (#3)

docs/architecture/system-overview.md

@@ -34,7 +34,7 @@ foreground of the `cron` process. Each scheduled invocation calls
 | `tenbackward.config`     | Loads `.env` + process env, applies defaults, validates schema, produces a typed `Config` dataclass.       |
 | `tenbackward.logging_setup` | JSON formatter (one log record per line), secret redaction, and `log_startup` / `log_error` / `log_run_summary` helpers. |
 | `tenbackward.state`      | Read/write of `posted.json` (atomic temp-file replace).                                                    |
-| `tenbackward.blog`       | Stubbed blog clone/update helper (lands in a follow-up job).                                              |
+| `tenbackward.blog`       | GitPython `ensure_repo()` — clones the configured blog repo on first run, fast-forwards it via `pull --ff-only` thereafter, with exponential-backoff retries on transient network errors. |
 | `/etc/cron.d/tenbackward`| Rendered cron file. One daily line that `cd /app` and runs `python -m tenbackward`.                        |
 
 # Communication & Wiring
@@ -80,7 +80,7 @@ foreground of the `cron` process. Each scheduled invocation calls
 | `/repo/src/tenbackward/config.py`   | Env merging, defaults, validation, `Config` dataclass, `ConfigError`.            |
 | `/repo/src/tenbackward/logging_setup.py` | JSON formatter, secret redaction, structured event helpers.                 |
 | `/repo/src/tenbackward/state.py`    | `posted.json` read/write with atomic temp-file replace.                         |
-| `/repo/src/tenbackward/blog.py`     | Stubbed blog clone helper (not yet wired into the pipeline).                    |
+| `/repo/src/tenbackward/blog.py`     | `ensure_repo()` — GitPython clone + fast-forward pull with `2^n` retry/backoff; raises `BlogRepoError` on local modifications or exhausted retries. |
 | `/repo/.env.example`                | Canonical list of environment variables.                                        |
 | `/repo/docker-compose.yml`          | Service definition; binds env vars from the host `.env`.                        |
 | `/repo/Dockerfile`                  | Builds the runtime image.                                                       |