docs(architecture): ADR-016 — native session preamble (review 001 A-3)

- new ADR-016: channels open-op params pinned as {repo, service}
  (channels/git/sub, additionalProperties: false); direct-ALPN GitAdapter
  parses the git-daemon request line (POC-1-verbatim grammar, capture-
  backed); session tuple gains the service dimension on both substrates;
  GitSession mirrors the shapes; version deliberately stays out of the
  preamble (service fully determines the state machine)
- amend ADR-002/005/010 (tuple, substrate inputs, open-op params pin) and
  transport.md/doors.md/overview.md/backend.md accordingly
- add the ADR-016 wire shapes to OQ-03's freeze inventory; note in
  AGENTS.md convention 9 that the alkgit-specific framing now exists and
  is pinned
- review 001: A-3 marked resolved

verification: cargo test, clippy -D warnings, fmt --check, doc — clean

docs/architecture/README.md

@@ -24,7 +24,10 @@ are the publish-freeze timing (OQ-03, a release decision), sha256
 policy (OQ-05, deferred on ecosystem need), and the grant-key identity
 namespace (OQ-16, deferred on the first cross-assembly deployment —
 blocks nothing in v1). ADR-015 resolved
-review 001's A-1 (the repo-op gate) with the `manage` grant tier.
+review 001's A-1 (the repo-op gate) with the `manage` grant tier; ADR-016
+resolved its A-3 (the native session preamble — `{repo, service}`
+open-op params, the git-daemon request line on the direct path, and the
+service dimension in the session tuple).
 
 ## Architecture Documents
 
@@ -55,6 +58,7 @@ review 001's A-1 (the repo-op gate) with the `manage` grant tier.
 | [013](decisions/013-receive-pack-state-machine.md) | receive-pack state machine (V0-framed push, thin-pack, unpack-first CAS) | Accepted |
 | [014](decisions/014-v2-negotiation-ack-loop.md) | V2 negotiation ack loop (no `ready`, wait-for-done stays) | Accepted |
 | [015](decisions/015-manage-grant-and-op-gate.md) | Manage grant tier + repo-op gate (admin scope OR manage grant) | Accepted |
+| [016](decisions/016-native-session-preamble.md) | Native session preamble (`{repo, service}` params, request line, service in tuple) | Accepted |
 
 ## Open Questions