Docs: consolidate active planning and archive historical material

The planning package had grown to where a new agent could not tell what was
authoritative. Phase 0 execution prompts sat beside the specification; four
completed milestone reports sat beside the current one; and upstream AI-DnD's
own `plan/` build log and `docs/` project site still described a hosted,
scripted, multi-user product with accounts — every screenshot in it showed a
Scripts tab and a Sign up button, none of which has existed since M2.

`planning/archive/` now holds the history and says so in its own README:
`phase0/` for the research that chose AI-DnD, `milestone-reports/` for M1 and
M2, `decisions/` for ADR 008, the Phase-0-before-build gate Phase 0 satisfied.
`planning/reports/` holds only the current milestone's report, because that is
the one M4 planning has to read; it moves to the archive when M4's replaces it.

Deleted rather than archived: the Phase 0B execution prompts and the
handoff/status/summary documents, the Phase 0A discovery and triage reports,
upstream's `plan/` and `docs/` trees, and `frontend/README.md`, which was Vite's
template boilerplate. All of it is in Git history, and the two recommendation
reports carry every conclusion the deleted research reached.

Archived documents are kept verbatim. Paths written inside them point at where
those files were when the document was written, which is the point: an evidence
record that has been quietly edited is no longer evidence.

Active documentation is corrected where it pointed at the removed trees or
described removed capability as present. `DEVELOPMENT.md`'s "things M1 did not
touch" list had gone stale at M2 and claimed QuickJS scripting was still tested;
its test count was 604 against an actual 638. `README.md` loses the upstream CI
badge, which reported upstream's pipeline rather than this fork's, and a
reference to `backend/app/worldstate/engine.py`, a file that does not exist.
`planning/README.md` is rewritten as the documentation index.

New: `planning/PROJECT-SOURCES.md` and `planning/project-sources.txt`, the
manifest of what belongs in the ChatGPT project's Sources.

Source comments referring to the deleted trees are reworded; no behaviour
changes. 638 backend tests pass, frontend lints and builds, and a reference scan
over all 48 tracked Markdown files reports no unresolved path in active
documentation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NCbwH7yLGKsj1rhXXzKSCu

backend/tests/test_chat.py

@@ -4,7 +4,7 @@ Most of this file used to be about the shared demo key: an access gate on a
 "power user" email allowlist, and a pinning rule that stopped a public visitor
 reaching paid models on a server-funded key. M2 removed the hosted deployment
 those defended, so the rules they tested no longer exist to be tested. See
-`planning/reports/M2-*` for the accounting.
+`planning/archive/milestone-reports/M2-*` for the accounting.
 
 What remains is what the page still does: stream a reply from the configured
 model, honour a system prompt and a per-request model override, and refuse a

backend/tests/test_state_revert.py

@@ -1,5 +1,4 @@
-"""Tests for undo and retry rolling back the shared `world_state`
-(plan/11-state-revert-and-retry-fix.md).
+"""Tests for undo and retry rolling back the shared `world_state`.
 
 The state being rolled back was the scripting engine's `script_state` until
 M2 removed campaign scripting. The machinery under test — `attempts.restore_state`,

backend/tests/test_story_tree_baseline.py

@@ -15,7 +15,7 @@ This file must pass unmodified through SP1 (schema), SP2 (branch clause),
 and SP3 (memories on nodes). If a change here looks necessary in one of
 those subphases, the change is wrong, not the test. SP4 is the first
 subphase allowed to move it, and only for the variant-count semantics
-called out in plan/14.
+the story-tree migration changed.
 """
 import pytest
 from fastapi import Depends

backend/tests/test_tree_migration.py

@@ -31,8 +31,7 @@ from app.main import app
 # The three tables as they stood at schema 45, frozen. This is a snapshot
 # of a past schema. It must not be updated to track `models.py`, because
 # the whole point is that it lacks what SP1 adds. This DDL uses SQLite
-# syntax only. The migration's Postgres half is exercised against a real
-# server at deploy time (see plan/14).
+# syntax only.
 PRE_TREE_DDL = (
     """
     CREATE TABLE adventures (