docs(agents): drop the fact-placement convention from AGENTS.md

[lesnik512]

Why

AGENTS.md carried a per-repo copy of a doc-placement convention: a four-homes table, an admission check to run "before writing a line anywhere", an ADR row prescribing NNNN-slug.md and revisit triggers, and a ratchet rule.

Doc placement is a domain-modeling convention. Restating it in every repo means ~25 copies that can drift from it, in a file loaded on every turn — the failure the removed text itself named: "it is always loaded, so a line that restates a docstring… costs every turn and rots in two places at once."

The local ADR rule was also weaker than the convention it duplicated: it had no reversibility gate, so any non-obvious rejected alternative qualified.

Piloted and merged in lite-bootstrap (#188); this applies the same cut here.

Design

Removed:

• the Where a fact goes section in full — four-homes table, the "before writing a line anywhere" admission check, "prose about mechanism has no home", the ratchet rule
• Workflow's "the spec for a change is its PR body" rule, the "two things outlive the PR" ADR sentence, "there is no change file and no lane to choose", and "there is no separate truth-home directory"
• Architecture's "behaviour detail has no prose home… run the admission check" tail, where present

Kept:

• the INVARIANT: docstring shape, with its ADR and lychee clauses trimmed
• one Workflow fact: unscheduled work becomes a GitHub issue
• every other section, unchanged
• inline docs/adr/ citations inside architecture prose. Those cite a specific decision at the point it is relevant; they are not the convention being dropped.

No replacement pointer is added, by design.

Non-goals

• No ADR is changed, removed or renumbered. docs/adr/ is untouched.
• No code or test changes.

Verification

The remaining docs/adr/ mentions in this file, if any, are inline citations to specific decisions, not a rule about when to write one.