feeders: stage beside the palace, not in ~/.cache; document Phase 1 exposure

Staging default moves out of ~/.cache to <palace-root>/pi-stage (pi) and
<palace-root>/opencode-stage (opencode), resolved with mempalace's own
palace-path precedence ($MEMPALACE_PALACE_PATH -> $MEMPAL_PALACE_PATH ->
~/.mempalace/config.json -> ~/.mempalace/palace), then dirname.

Why: the convos miner keys dedup on the *staged* path, so a wiped stage plus a
sync scoped to include it prunes the drawers mined from those sources --
deleting memories, not a cache. Under ~/.cache that state was reachable by
anything treating a cache as disposable. Staging inside the palace makes the
coupling structural: the stage cannot be wiped without touching the palace
itself. Overrides ($MEMPALACE_PI_STAGE / $MEMPALACE_SESSION_STAGE, --stage) are
unchanged. Note the old default had never been created on any host, so this
closed a latent hazard, not a live one.

Measured, and the docs now claim only this much: sync prunes only within the
scope it is given -- wing-only, 1299 scanned / 1299 out of scope / 0 removed;
scoped at the palace root, 651 kept / 648 out of scope. The previous blanket
"sync prunes every drawer" wording overstated it, which is a liability: the next
reader disproves the overstatement and discards the real constraint with it.

Also in this change:
- cron log dir ~/.cache/mempalace-session -> ~/.cache/mempalace-logs. The stage
  left that namespace, so the old name now read as "the stage".
- AGENTS.md: the convos miner *does* check mtime (verified against upstream
  convo_miner.py); the previous "no mtime check" claim was wrong.
- smoke-test assertions use `mktemp -d` for --sessions-dir. One pointed at /tmp,
  which still held earlier synthetic transcripts, so a --dry-run exported a fake
  session into the real stage: --dry-run skips the mine, not the export.

docs/phase-1-exposure-runbook.md -- the newt/DNS/auth step that RFC 001 and the
synlig runbook leave open (runbook section 4, items 2 and 5). Port 8765 at /mcp,
newt targets 172.17.0.1, and the authentication is the single shared bearer
token (RFC 6.2, decided 2026-08-09) rather than per-device proxy users. The
latter cannot work today: mempalace validates exactly one token, and Pangolin's
SSO/PIN/password are browser-shaped while every client here is a headless
JSON-RPC POST -- enabling that protection breaks the clients it protects. The
per-device axis that *does* exist is the feeder's SSH key + per-device inbox.

New finding recorded there: a loopback bind does not merely 403 behind a tunnel
(already known, runbook 2.4) -- it also silently starts the server with no token
at all, because auto-minting is gated on the bind being non-loopback.

extensions/pi/README.md: the HTTP transport IS authenticated as of mempalace
3.6.0; the "sessionless and unauthenticated" note dated from the v1.3.0 era.
Closes the RFC section 8 Phase-0 hygiene item.

AGENTS.md

@@ -152,7 +152,15 @@ For `mempalace-docs`, test on a small repo (e.g. this one) first:
 - The companion skill lives at `~/.agents/skills/opencode-mempalace-bridge/SKILL.md` and is a **symlink into this repo**. Editing that file edits `SKILL.md` here. To propagate to Claude Code / Kiro, run `agents-sync` from [`cli_utils`](https://gitea.jordbo.se/joakimp/cli_utils).
 - The opencode DB path defaults to `~/.local/share/opencode/opencode.db`. Override via `$OPENCODE_DB` or `--db`.
 - The mempalace miner **skips symlinks** (as of v3.3.3 — `miner.py` line ~828). That's why the wrappers use `cp -p` / explicit file writes for staging, not symlinks.
-- The convos miner dedups on `source_file` path only (no mtime check). Staging filenames must be stable per session; deleting a staged JSONL forces a re-mine.
+- The convos miner dedups on `source_file` path **and** `mtime`
+  (`file_already_mined(..., check_mtime=True)` in upstream `convo_miner.py`).
+  A changed/grown transcript is detected, purged, and refiled — it is not
+  silently skipped. What must still be stable across runs is the *staged
+  path itself*: if a wrapper's staging dir is wiped, dedup has nothing to
+  compare against and `mempalace sync` will treat the vanished sources as
+  deleted and prune the drawers mined from them. (An earlier version of this
+  file claimed "no mtime check" — that was wrong; verified against
+  `convo_miner.py` 2026-08.)
 - The docs miner dedups on `source_file` path + `mtime`. That's why staging uses `cp -p` (preserves mtime).
 
 ## Colocated skill pattern