docs(kb): the TUI composed-row rule, and a trap in our own remote topology

PRs #145/#149. New gotcha 50: `fill_line` takes a `Keeps` because the three
rows sharing a line disagree about which side yields, and the one key table on
screen is the one derived from the keymap — with the import wizard as the
deliberate inverse, since its derived line comes from the mode keymap while
the real keys are per-step.

AGENTS.md gains the remote trap, and it is the orchestrator's fault: briefs
said "merge `main`", and because `origin`/`gitea` take `main` directly while
GitHub gets branches, `origin/main` runs ahead of `gh/main`. An agent doing
the habitual thing pulled four unmerged commits — another PR's whole diff —
into a branch based on `gh/main`. Nothing in CI can see it; the result
compiles and passes. Codex caught it. The window is transient, which makes it
worse: the hazard is absent exactly when you would think to check.

Closes improvise-t14d, improvise-rr8n and improvise-vgp4 — the last
stale-open, fixed by 94a3d17 two days before it was filed.

Measured on main at 78a7bbf: 1700 passed / 0 failed / 7 ignored over 38
binaries, longest single binary 0.69s. kb_consistent: TRUE, 792 facts.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

AGENTS.md

@@ -139,6 +139,21 @@ merges agent branches into its *local* `main` to integrate and run the gates —
 that is how conflicts get found early — but the push is a branch push, not a
 `main` push. `origin` and `gitea` are mirrors and take `main` directly.
 
+**So `origin/main` runs ahead of `gh/main`, and a branch destined for a GitHub
+PR must merge `gh/main` — never `origin/main`.** This is a trap in this repo's
+own topology rather than a slip: the two remotes are both spelled `main`, and
+nothing about `git merge origin/main` looks wrong. The session-completion
+protocol below literally says `git merge origin/main`, which is right for
+`main` itself and wrong for a PR branch. It cost a review round on PR #145,
+where four unmerged commits — including another PR's whole diff — were pulled
+into a branch based on `gh/main` and had to be rebuilt by cherry-picking.
+ChatGPT Codex caught it; nothing in CI could, because the result compiles and
+passes. The tell is a PR diff naming files the ticket has no reason to touch,
+so **read your own PR's file list before asking for review**. Note the window
+is transient — the two remotes are equal immediately after the orchestrator
+syncs — which makes it worse, not better: the hazard is absent exactly when
+you would think to check for it.
+
 This is easy to drift out of, because merging locally and pushing feels like
 finishing. It happened in this repo: 27 commits across six agent branches —
 including a new crate, a new protocol type, and a type moving between crates —