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, a ratchet rule, and an index of the repo's ADRs.

Doc placement is a domain-modeling convention. Restating it per 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: no reversibility gate, so any non-obvious rejected alternative qualified. That is how a since-dropped ADR rejecting a gRPC bootstrapper got written — trivially reversible, nothing built, and it would have been filtered before its argument was examined.

Design

AGENTS.md goes from 103 lines to 56. 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
• Architecture's "why four shapes are load-bearing" list and its five ADR links, plus the "Behaviour detail has no prose home… run the admission check" tail
• Workflow's "the spec for a change is its PR body" rule, "there is no change file and no lane to choose", and "there is no separate truth-home directory"

grep -i adr AGENTS.md now returns nothing.

Kept: Project Overview, Commands, Architecture's file-layout paragraph, Code style, Type checking, one Workflow fact (unscheduled work becomes an issue), and the INVARIANT: docstring shape.

Non-goals

• No ADR is changed, removed or renumbered. docs/adr/ and its nine records are untouched — only the prose that indexed them and prescribed when to write one.
• No test is changed here.
• No replacement pointer to the convention is added, by design.
• This is the pilot for the same edit across 24 other repos; those are separate PRs and none is open.

Companion change

modern-di enforces the INVARIANT: docstring shape with tests/test_invariant_census.py; that test is being dropped separately. The shape stays documented in AGENTS.md and unenforced by CI, checked at review time — which is what this file now says.

Verification

• grep -i adr AGENTS.md → no matches.
• No dangling cross-reference: the only pointer to Where a fact goes was the Architecture tail, removed in the same change.
• Trailing newline preserved for eof-fixer. Every removed markdown link was relative to a target that still exists, so the lychee gate is unaffected.