docs: merge the upsert ADRs, drop replaced and non-ADR records, renumber

[lesnik512]

This PR tidies up docs/adr/ and changes the rule for replaced records.

Rule change

docs/adr/README.md used to keep a replaced ADR in place with superseded_by frontmatter. Now the reasoning that still holds moves into the new record and the old file is deleted, since git history keeps it. With that, every file in the directory is a current decision, so ADRs carry no frontmatter.

Records

Before	After	Change
0005 upsert via DuplicateKeyError recovery	deleted	Replaced by the upsert. Its lasting point, that a pre-read can't make the insert safe, moved into the new 0011.
0012 message send is one upsert, 0013 direct chat is one upsert	0011 idempotent creates are one upsert	Merged: same statement, same xmax check, same rejected options. This also removes the duplicate 0012.
0009 coverage exclusions	deleted	Fails "hard to reverse". The rule now sits under Conventions in AGENTS.md.
0006 idempotency scoped per chat	0005	Trimmed the bug history to two sentences.
0007 read-marker integrity	0006	Kept the API behaviour: reject ids from other chats, silent no-op for a lower id. The IS DISTINCT FROM and monotonic details now live in INVARIANT: docstrings on test_system_messages_count_as_unread and test_marking_read_is_monotonic. The reason GREATEST runs inside the UPDATE was already in ChatMembersRepository.mark_read's docstring.
0008, 0010, 0011, 0012 pool pre-ping	0007, 0008, 0009, 0010	Renumbered only. The logout ADR's link to the actor-id ADR moved with it.

Nothing outside docs/adr/ cites an ADR by number. docs/agents/domain.md cites ADR-0001, which keeps its number.

Verification

Run locally: just adr-check, ruff, ty, and the full suite (141 passed, 100% coverage).