Stop the plan's opening from describing a greenfield app

The first fifteen lines of IMPLEMENTATION_PLAN.md were the most misleading text
in the repository. They told an agent to work on branch feat/chart-engine, which
does not exist; to build M0 through M5 and stop for feedback, all of which
shipped days ago; and that the repo was a placeholder app with a toy /api/hello
endpoint to delete. It is the first thing anyone reads.

Replaced with what is true: the document is mostly history now, current work
starts from AGENTS.md, main deploys to production by design, and §16 onward is a
dated log that is the most useful part of the file for anyone debugging.

Also adds docs/archived/ with the convention written down, though nothing has
earned a place in it yet — feature_undo.md and mobile_enhance.md are designs not
yet built rather than dead ones. Archived documents stay tracked: gitignoring
them would delete them from the repository, which loses the history that makes
them worth keeping in the first place.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/IMPLEMENTATION_PLAN.md

@@ -15,20 +15,27 @@ See [§14](#14-why-execution-is-out-of-scope) for why, and for the seam left beh
 
 ## 0. Start here
 
-**Read §1, §2.1, §6, and §13 before writing anything.** The rest can be read as you
-reach each milestone.
-
-**Work on a branch — do not push to `main`.** `main` is wired to a Forgejo webhook that
-triggers a Coolify production deploy at <https://chart.amow.com>. Pushing to main ships
-whatever you wrote. Branch: `feat/chart-engine`.
-
-**Build order is M0 → M1 → M2 → M3 → M3.5 → M4 → M5.** Stop after M5 and get feedback;
-M6+ are separately scoped. No API keys are required for any of M0–M5.
-
-**Existing repo state:** a placeholder FastAPI + Vue 3 (CDN, no build step) app.
-`main.py` serves `static/index.html` and two toy `/api` endpoints. The serving and
-deploy wiring is correct and should not be redesigned — extend it. The toy `/api/hello`
-endpoint and its frontend button can be deleted.
+> **This document is now mostly history.** M0–M10 are built and deployed. The
+> build order, branch instructions and "existing repo state" that used to open
+> this file described a greenfield app and were actively misleading by August
+> 2026, so they are gone. What remains below is the reasoning behind decisions
+> already made — read it to understand *why* something works the way it does,
+> not to find out what to build.
+>
+> **For current work, start with `AGENTS.md`**, which every agent loads
+> automatically. It points at the live planning documents:
+> `docs/NEXT_STEPS.md` for the short list, `docs/async_refactor.md`,
+> `docs/multi_user.md`, `docs/feature_undo.md` and `docs/mobile_enhance.md` for
+> designs not yet built.
+>
+> **`main` deploys to production.** A push triggers a Forgejo webhook and
+> Coolify rebuild of <https://chart.amow.com>. That is the intended workflow now,
+> not an accident to avoid — but it means every push is a deploy, and a deploy
+> restarts the market stream.
+>
+> §16 onward is a dated log of problems and their resolutions. It is the most
+> useful part of this file for anyone debugging: most entries record something
+> that looked like one bug and was another.
 
 ### Dependencies to add
 

docs/archived/README.md

@@ -0,0 +1,16 @@
+# Archived documents
+
+Superseded designs and plans, kept because the reasoning is often still useful
+and the history is worth more than the disk space.
+
+**Agents: skip this directory unless you are asked about it, or you are tracing
+why a decision was made.** Nothing here describes how the app currently works.
+
+These stay tracked in git rather than gitignored — ignoring them would delete
+them from the repository, which loses exactly the history that makes them worth
+keeping.
+
+When archiving something, add a line here saying what replaced it.
+
+_(Empty so far. Nothing has genuinely died yet: `feature_undo.md` and
+`mobile_enhance.md` are designs not yet built, and `NEXT_STEPS.md` is current.)_