Say how work divides when several agents run at once (#746)

[Rafael-SOWNet]

AGENTS.md has a working practice for one agent: branch, failing test first, run everything,
record a changed answer, read the thread before merging. It says nothing about the case this
repository is actually in, which is several agents working at the same time.

That case has failure modes the single-agent rules do not cover, and they are not the ones people
expect. Being slow is not the problem. Two agents building the same abstraction twice is, and so is
one reverting the other — and neither shows up as a merge conflict. Git warns about a second edit to
the same line; it does not warn about a second design.

What the new section settles

Ownership is claimed before writing, and by component rather than by issue number. Two issues in
one file are one task; one issue across two subsystems is usually two.

A merged PR is not the synchronisation primitive. Dependent work may build on an unmerged branch
once that branch's design is stable, and independent work never waits at all. The real constraint
runs the other way: a foundational abstraction still being argued about should not have a pile of
downstream work on it yet. The mechanical trap git does not warn about is recorded alongside it — a
branch cut on top of another branch merges into that branch, reads MERGED, and leaves master
without the code.

The repository is the source of truth, not the tracker and not another agent's description of
the code. An open issue does not mean the functionality is missing and a closed one does not mean it
is complete; both have been wrong here, which is why the 2026-08-23 audit of #746 struck items
through against the code rather than against their issue state.

When two streams need the same foundational code, parallel implementation of that part stops
until one design is agreed. Throughput gets no exception from the rule at the top of the file.

It also records the roadmap-execution loop — audit, dependency graph, critical path, run everything
unblocked concurrently, integrate, repeat — because a large issue is a dependency graph written down
as a list, and the list order is not the execution order.

Scope

AGENTS.md only, +114 lines, one new ## Agentic development section inserted between Working
practice and Write for the reader, briefly. Nothing existing is reworded and no other section
moves — git diff --stat is 1 file changed, 114 insertions(+).

No code, so no behaviour change and no BREAKING-CHANGES.md entry.

Merge order

AGENTS.md is also edited by #1014 and #1016. Whichever of the three merges second and third pays
one mechanical conflict round each; the three edits are in different sections, so the resolution is
mechanical.

Part of #746.