PIN=auto — seed the pin from the live usage history

PIN=auto resolves to <model>/.coli_usage, the history that serve mode
appends after every turn, so each restart's hot-store placement follows the
accumulated REAL workload instead of a frozen one-shot profile; stats.txt is
the fallback for a virgin model dir, and with neither present the run simply
starts unpinned. Same magic-value convention as PIN_GB=all (#80); explicit
paths and the AUTOPIN flow are untouched (PIN set skips AUTOPIN as before).
This lifts deployment entrypoints' "prefer .coli_usage over stats.txt" shell
plumbing into the engine, plus the ENVIRONMENT.md row.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ENVIRONMENT.md

@@ -47,7 +47,7 @@ Format: `VAR` — default — effect.
 | `CAP_RAISE` | `1` (on) | Let the engine raise the expert-cache cap above `topk` when RAM allows (bigger batches). `0` fixes the cap. |
 | `PREFETCH` | `0` | Prefetch depth for streamed experts. |
 | `COLI_MMAP` | `0` | `mmap` the weights instead of read()-ing into slabs. |
-| `PIN` | unset | Path to a `.coli_usage` file; pins the hottest experts into a resident "hot store" at startup. |
+| `PIN` | unset | Path to a `.coli_usage`/stats file; pins the hottest experts into a resident "hot store" at startup. **`PIN=auto`** seeds from the model dir's live `.coli_usage` (appended after every turn, so each restart's pin placement follows the accumulated real workload) with `stats.txt` as the fallback for a virgin model dir; neither present → no pin this run. |
 | `PIN_GB` | `10.0` | Size budget (GB) for the pinned hot store when `PIN` is set. |
 | `AUTOPIN` | `1` (on) | Auto-pin the hot store from usage history once ≥5000 selections are recorded. |
 | `REPIN` | `0` (off) | Live re-pin the hot store every N emitted tokens (RFC). |