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

PROVENANCE.md

@@ -26,9 +26,11 @@ import is a merge of the pinned commit with `--allow-unrelated-histories`, so:
 
 - `git log d72f7c1bda0f34fccd84afb7a25c34eb01c901de` shows the real upstream
   history, not a squashed snapshot;
-- upstream paths are unchanged (`backend/`, `frontend/`, `docs/`, …), so a
-  later upstream commit can still be fetched and cherry-picked against
-  matching files;
+- upstream code paths are unchanged (`backend/`, `frontend/`, …), so a later
+  upstream commit can still be fetched and cherry-picked against matching
+  files. Upstream's own documentation trees, `plan/` and `docs/`, were removed
+  on 2026-09-03: they described the hosted, scripted, multi-user product this
+  fork is not. They remain in this repository's history and in upstream;
 - the planning package that predates the fork keeps its own history on the
   other parent of the merge.
 
@@ -104,7 +106,7 @@ unchanged. They are not product functionality and nothing reads or writes them.
 ## What this fork changed in Milestone M1
 
 Nothing was removed from upstream. The changes are the offline/locality
-hardening M1 called for; see `planning/reports/M1-BASELINE-REPORT.md` for the
+hardening M1 called for; see `planning/archive/milestone-reports/M1-BASELINE-REPORT.md` for the
 evidence.
 
 - `backend/app/context/encoding.py` (new) and `backend/app/context/builder.py` —