fix(init): split --auto-mine from --yes; show file-count estimate before mine prompt

Reviewer feedback on the previous commit flagged two real problems:

1. Overloading --yes to also auto-mine was a silent behaviour change for
   scripted callers. Today --yes only auto-accepts entities — making it
   ALSO trigger a multi-minute ChromaDB write breaks every script that
   currently runs `mempalace init --yes <dir>` for the fast non-interactive
   entity path. Add a separate `--auto-mine` flag instead. Combinations:

     mempalace init --yes <dir>              # entities auto, STILL prompt mine
     mempalace init --auto-mine <dir>        # prompt entities, skip mine prompt
     mempalace init --yes --auto-mine <dir>  # fully non-interactive

   --yes behaviour is now identical to pre-PR.

2. The mine prompt was firing without telling the user how big the job
   was. On a real corpus mine takes minutes-to-tens-of-minutes; hitting
   Enter on default-Y with no size cue is a footgun. Show a one-line
   estimate computed from scan_project (the same walk we hand into mine)
   BEFORE the prompt:

     ~423 files (~12 MB) would be mined into this palace.
     Mine this directory now? [Y/n]

   The estimate uses a single corpus walk: scan_project's output is
   passed into mine() via a new optional files= kwarg, so we never walk
   the tree twice.

Tests: replaced the old "--yes auto-mines" assertion with a regression
guard that --yes alone STILL prompts; added coverage for --auto-mine
alone, --yes --auto-mine together, and the pre-prompt estimate line.

CHANGELOG.md

@@ -10,7 +10,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 
 ### Added
 
-- **`mempalace init` now prompts to mine the same directory.** After entity confirmation, room detection, and gitignore guard, `init` asks `Mine this directory now? [Y/n]` (default yes) and runs `mine()` in-process if accepted. `--yes` skips the prompt and auto-mines, so non-interactive invocations are still hands-off. Declining prints the exact `mempalace mine <dir>` command for later. Removes the "remember to type the next command" friction that existed because the rooms + entities just set up are almost always mined immediately. (#1181)
+- **`mempalace init` now prompts to mine the same directory.** After entity confirmation, room detection, and gitignore guard, `init` shows a one-line scope estimate (e.g. `~423 files (~12 MB) would be mined into this palace.`) computed from its existing corpus walk, then asks `Mine this directory now? [Y/n]` (default yes) and runs `mine()` in-process if accepted. The estimate fires before the prompt so users on a real corpus aren't surprised by a minutes-long ChromaDB write. Declining prints the exact `mempalace mine <dir>` command for later. (#1181)
+- **New `--auto-mine` flag on `mempalace init`** for the non-interactive path (`mempalace init --auto-mine <dir>` skips the mine prompt and runs mine directly). `--yes` retains its existing scope of entity auto-accept only and still prompts for the mine step, so existing scripted callers see no behaviour change; combining `--yes --auto-mine` gives a fully non-interactive setup. (#1181)
 - **Cross-wing topic tunnels.** When two wings have confirmed `TOPIC` labels in common (the LLM-refine bucket from `mempalace init --llm`), the miner now drops a symmetric tunnel between them at mine time so the palace graph reflects shared themes (frameworks, vendors, recurring concepts). Tunnels are routed through the existing `create_tunnel` storage so they share dedup and persistence with explicit tunnels. Topic tunnels are stored under a synthetic `topic:<name>` room and tagged with `kind: "topic"` on the stored dict — this keeps them distinct from literal folder-derived rooms of the same name (a wing with both an `Angular` folder room and an `Angular` topic tunnel no longer collides at `follow_tunnels` read time) and gives LLMs scanning `list_tunnels` a visible discriminator. Threshold is configurable via `MEMPALACE_TOPIC_TUNNEL_MIN_COUNT` env var or `topic_tunnel_min_count` in `~/.mempalace/config.json` (default `1`). Manifest-dependency overlap and per-topic allow/deny lists remain out of scope. (#1180)
 
 ### Bug Fixes