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/overview.md

@@ -29,8 +29,8 @@ Single crate `alkgit`:
 
 | Half | Contents | POC evidence |
 |---|---|---|
-| Producer | `GitAdapter` (`alk/git` ALPN via alkcall `ProtocolHandler`), channels `register_openable` (repo id in open-op params — the negotiation + ACL point) | POC-1 verbatim |
-| Consumer | `GitSession` typed client (`connect_direct`, `open_via_channels`) — the replication/mirroring primitive for alknet | new, small (TtySession analog) |
+| Producer | `GitAdapter` (`alk/git` ALPN via alkcall `ProtocolHandler`; parses the ADR-016 request-line preamble), channels `register_openable` (`channels/git/sub` — open-op params `{repo, service}`, the negotiation + service selector + ACL point; ADR-016) | POC-1 verbatim |
+| Consumer | `GitSession` typed client (`connect_direct`, `open_via_channels`) — sends the same ADR-016 preamble shapes | new, small (TtySession analog) |
 | Substrate | duplex session + stateless request/response layer (ADR-005); wire framing, V2 state machines (ADR-003) | POC-1, POC-3 |
 | Backends | `GitRegistry` (+ write supertrait), `GitRefs`, `GitPackGen`, `GitPackIngest` traits; impls behind the default-on `gix` (engine) and `registry-file` (records) features | POC-2 (gix impl) |
 | Management ops | `git/repo/*` call ops over `GitRegistryStore` (ADR-012 §3) — the JSON half alongside the `alk/git` open op (first dual-kind payload; ADR-012 §5) | thin over the store trait |
@@ -79,7 +79,7 @@ multi-round negotiation are design-complete against real-client captures
 | ADR | Decision | Summary |
 |---|---|---|
 | [001](decisions/001-crate-decomposition.md) | Crate decomposition | **superseded by ADR-010** |
-| [002](decisions/002-front-door-blind-core.md) | Session boundary | (identity, repo, stream, limits) — unchanged, load-bearing |
+| [002](decisions/002-front-door-blind-core.md) | Session boundary | (identity, repo, service, stream, limits) — unchanged, load-bearing; service per ADR-016 |
 | [003](decisions/003-protocol-v2-first.md) | V2-first protocol | V2-only fetch both doors; push is V0-framed by upstream design (ADR-013); honest advertisement |
 | [004](decisions/004-pack-pipeline.md) | Pack pipeline | `data::output` gen / `data::input` ingestion |
 | [005](decisions/005-session-substrate-types.md) | Substrate types | duplex + stateless APIs over one state machine |
@@ -93,7 +93,7 @@ multi-round negotiation are design-complete against real-client captures
 | [013](decisions/013-receive-pack-state-machine.md) | receive-pack | V0-framed push machine, thin-pack acceptance, unpack-first CAS |
 | [014](decisions/014-v2-negotiation-ack-loop.md) | Negotiation | ack loop, `common_haves` seam, no `ready` |
 | [015](decisions/015-manage-grant-and-op-gate.md) | Manage grant + op gate | `manage` tier, admin-OR-manage gate, create seeds manage |
-| [015](decisions/015-manage-grant-and-op-gate.md) | Manage grant + op gate | `manage` tier, admin-OR-manage gate, create seeds manage |
+| [016](decisions/016-native-session-preamble.md) | Native session preamble | `{repo, service}` open-op params, request-line preamble, service in the tuple |
 
 ## Open Questions
 
@@ -101,7 +101,9 @@ Key questions tracked in [open-questions.md](open-questions.md):
 
 - **OQ-03**: publish-time API freeze inventory (the `git/repo/*` op set
   and the trait family enter it; ADR-012, ADR-013/014's trait additions;
-  the ADR-015 three-action grant shape must land in it).
+  the ADR-015 three-action grant shape must land in it; the ADR-016
+  native preamble wire shapes — `{repo, service}` params schema and the
+  request-line grammar — entered it).
 - **OQ-05**: sha256 policy (deferred(scope), low).
 - **OQ-16**: grant-key identity namespace (deferred(scope); blocks
   nothing in v1 — ADR-015 §7).