docs: living agent engineering guide + user intro; wire CLAUDE/AGENTS to it

docs/AGENT_ENGINEERING_GUIDE-RUNNING.md is the agent-maintained living
engineering guide: self-describing maintenance protocol (correct the body
in-session when reality diverges; append a concise datetime-stamped
session entry after major work; promote durable lessons into the body),
then the distilled knowledge - what Flux is technically, the twin-engine
rule with the shared-core/parity-gate table, data model + persistence
invariants, renderer architecture, hard rules, the verification system,
recipes for common work (new feature / new verb / new IPC channel /
format change / perf), known traps (Svelte 5 legacy reactivity,
measurement, environment, bundle), deliberate deferrals so agents do not
"fix" intentional decisions, and the session log seeded with the fortify
engagement entry.

docs/INTRO.md is the non-technical user introduction: one page on
projects/modes/autosave/agents plus a half-page of core concepts and
controls per module (Paper, Figure, Slide, Reader, Library), grounded in
the actual keybindings.

CLAUDE.md now directs every session to the guide first; new AGENTS.md
does the same for other agent harnesses.

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

Author: kortdriessen

AGENTS.md

@@ -0,0 +1,14 @@
+# Flux — instructions for AI agents
+
+Two documents govern all agent work in this repo. Read both before doing anything substantive,
+in this order:
+
+1. **`docs/AGENT_ENGINEERING_GUIDE-RUNNING.md`** — the living engineering guide (architecture,
+   the twin-engine rule, hard rules, verification system, recipes, known traps, session log).
+   It is agent-maintained: it contains its own instructions for how to correct it and how to
+   append a session-log entry after major work. Follow those instructions every session.
+2. **`CLAUDE.md`** — the short list of absolute invariants (locked editing feel, config paths,
+   repo etiquette, verification conventions).
+
+If anything in this file, `CLAUDE.md`, or the guide contradicts reality, the guide's maintenance
+rules apply: fix the document in the same session and note it in your session-log entry.

CLAUDE.md

@@ -1,5 +1,16 @@
 # Flux — notes for agents
 
+## Read the engineering guide first (every session)
+
+Before doing substantive work in this repo, read
+**`docs/AGENT_ENGINEERING_GUIDE-RUNNING.md`** in full. It is the living,
+agent-maintained engineering guide: architecture, hard rules, the verification
+system, recipes, known traps, and a session log. It also tells you how to
+UPDATE it — corrections when you find something outdated, and a concise
+datetime-stamped session entry after any major work. Treat that document as
+part of your instructions for every session; this file is only the short list
+of absolute invariants.
+
 ## The Paper editor's editing feel is LOCKED
 
 The cursor/typing/editing feel of the manuscript editor