docs(agents): generalise the shared-stash rule to all shared ref namespaces (#12103)

AGENTS.md §9 gains the general four-namespace rule beside the existing stash
rule: a worktree isolates the checkout and exactly HEAD, refs/bisect,
refs/worktree and refs/rewritten, and nothing else. refs/stash is reframed as
one case; refs/remotes/* and FETCH_HEAD are added as the two further instances
that have cost work, with FETCH_HEAD stated per CHECKOUT (last fetch wins),
never as "shared by every worktree" — that wording was measured false on
git 2.43. Four practices and the deliberate no-hook decision land with it.

The footer block gains the one clause it was missing: a PR-body edit also
APPENDS a platform bare footer, so the floor is the author's session-URL footer
plus one platform bare, and normalisation cannot converge below it.

Both files are at zero-headroom ratchet ceilings, so the additions are paid for
by compressing §9 narrative in place — no ceiling raised, no rule, gate name,
command or measured number removed.

Closes #11946, closes #11273

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-zhuang