Write down what the llama.cpp work actually does

Phase 4 is no longer deferred and phase 5 is exercised, so the status
section says so. The two decisions worth not undoing by accident get
named: the conversation lives in the transcript rather than the driver,
and a llama session is refused on an ssh host rather than half-working.

Also the local testing recipe, including the trap that cost me twenty
minutes -- a 2-bit quant produces fluent nonsense that reads exactly like
a broken driver, and llama-cli on the same file is how to tell the two
apart.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017xn8nHw1tw1R6PtiY1eEtw

PLAN.md

@@ -222,6 +222,28 @@ turn. Claude's dialect: a `user` message on stdin mid-stream; pi's: `steer`.
   endpoint changes.
 - pi's session JSONL gives resume-after-restart, same as Claude's.
 
+### Models (built 2026-08-28)
+
+Bryan asked for listing and downloading models from HuggingFace and running
+them with different parameters, which makes model management part of the
+feature rather than something done by hand beforehand.
+
+- **A download belongs to the model, not to the request.** Keyed by
+  `owner/repo/file.gguf` and owned by the server, so a second device can
+  watch one it did not start, and so an hour-long fetch survives a phone
+  locking its screen. Every run has an id and its outcome outlives it,
+  because "not downloading" otherwise means finished, never started, or
+  someone else's run ended while you were away.
+- **Progress is measured.** `total` is Content-Length, or Content-Range's
+  last field on a resumed request, and absent when the server says
+  nothing — never an estimate.
+- **Resume is guarded by identity, not by hope.** A partial carries the
+  ETag it was written against; a mismatch discards it. `If-Range` would be
+  the tidy mechanism but HuggingFace's CDN ignores it (probed
+  2026-08-28). The published sha256 is checked before the file is renamed.
+- Parameters reach a driver as an untyped `params` map on the session, so
+  the shared schema does not grow llama.cpp's vocabulary.
+
 ### llama-server management
 
 `config.ron` lists **models** (name → GGUF path or llama-server args, per