Plan the readability refactor, and clear the tree for it

Phase 17 splits the four files that hold most of the code, finishes the
schema migration SP8 left half done, and stops the published guide from
drifting away from its Markdown source. `plan/17-refactor.md` carries the
plan and the progress table, and `plan/STATUS.md` points at it.

Stage 0 is hygiene only. Both abandoned worktrees are gone, which freed
about 104 MB. Removing `sp7-tree-ui` needed one extra step: a Vite dev
server had been running out of it since 2026-08-18, holding
`frontend/.vite` open and owning port 5173, and serving a tree 54 commits
behind `main`. The three stale `.db` files are deleted; `data.db` is not.

`AIDND_TRUSTED_PROXY_HOPS` is now documented. It was read at `limits.py:55`
and named in no `.env.example`, README, or blueprint. It sets how many
proxy hops the rate limiter trusts in `X-Forwarded-For`, so a deployment
that adds a hop without setting it gets the bucket-rotation bypass back.

The 19 squash-landed branches are still there. `git branch -D` is blocked
by the permission classifier; the verified command is in the plan file.

549 tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014Dix4oGV3njgWRdu7P9t6r

plan/STATUS.md

@@ -78,6 +78,10 @@ needed; nothing requires reading a row of anyone's story.
 
 ## Pick up here
 
+**`plan/17-refactor.md` is the active phase.** It carries its own progress table, which
+is the first thing to read when you pick the work back up. It runs in six stages, and
+SP8 below is stage 3 of it. Nothing in it changes what the app does.
+
 **`plan/14-phase-story-tree.md`, SP8 — drop the legacy columns.** SP8 was gated on the
 tree being proven live, and it now is: SP9 merged, and production answers `/api/health`
 with the tree schema in place. SP8 drops `index`, `variants`, `variant_index`,