Fleet Studio B2b: wire the Enable-control act end-to-end + re-home it over the composer (#1551)

[jeonghun-jj-lee]

Closes #1551. Base: feature/free-tier-fleet.

The last mile of B2b, surfaced by live verification after #1547 merged: on a self-owned remote peer-studio session, Enable control was a no-op and sat over the titlebar. This wires the act and re-homes the affordance.

What was broken

• The enable act was wired to nothing. The app posted amicode.fleet.enableControl, but that command was not on the bridge allowlist and no extension command handled it; enableSelfOwnedControl / evaluateControlBootstrap had zero live callers. Fleet Studio B2b · slice 4 — Control UI: enable, driving banner, fail-closed affordances #1544 shipped the state channel and Fleet Studio B2b · slice 2 — Observation write plane: control-gated remote writes #1542 the write plane, but the act that mints the grant was never connected.
• Placement. The affordance was a fixed top-right Portal over the titlebar.

What this does

• DEFECT 1 — wire the act (976b065). A payload-carrying fleet-enable-control bridge envelope ({ ownerMachineId, sessionID } — the bare command lane can't carry a target) → a new vscode-free handleEnableControlRequest (confirm-first) composing establishManagementVerified + enableSelfOwnedControl. Activation wires the ADR-0034-D4 native modal + the live fleet-peer provider / enroll-seeded authority / grant store. Grant persistence flips the projection: the next /amicode/fleet/sessions poll re-resolves amicode_control to interactive and lights the driving banner — the app never self-declares interactive (SoT stays the source).
• DEFECT 2 — re-home over the composer (392fe7c). New SessionComposerControlScrim wraps the composer slot in session.tsx: while a remote peer session's control is not held, the composer is blurred + inert (non-editable) and a centered CTA ("This session lives on <peer> — Enable control to drive it") posts the DEFECT-1 envelope. Held/local → no scrim.

Invariants held

• No-privilege-bleed: a shared (non-self-owned) peer mints nothing even on confirm (only the Fleet Studio B2b · slice 5 — Shared-peer control: request->approve handshake #1545 handshake, still inert).
• Command lane byte-identical (BRIDGE_ALLOWED_COMMANDS untouched; new payload kind added beside it).
• Cancelling the modal mints nothing; fail-closed on a standalone/unenrolled machine.
• Fleet Studio B2b · slice 4 — Control UI: enable, driving banner, fail-closed affordances #1544 AC5 regression guard intact — no control markers leak into the read-only sidebar.
• Additive-only; observation read/write planes and the projection shape unchanged.

Gates (director-independent re-run on this branch)

• Touched ext suites 221/221 (fleet_control_bootstrap 24, chat_bridge 66, sidebar_fleet_section 100, chat_panel 31).
• Full ext suite: 12 fail / 4934 pass — all 12 environmental (11 macOS-loopback/SSH transport + cli_gate, which is a fresh-worktree staging artifact and passes 12/12 on the base checkout). Zero failures in any touched file; zero regressions.
• Extension typecheck clean · additive-invariants (4/4 ADR-0027) · drift gate PASS (875 files) · overlay typecheck clean ×7 · app tests 37/37.

Deploy + verify (for JJ)

1. Rebuild the app bundle + binary from this branch and restart VS Code (the enable path rides the app↔extension bridge, not a new HTTP route).
2. Open "Free port 4096" → the composer is blurred with the CTA → click Enable control → native modal → confirm → the driving banner lights and the composer unblurs; a prompt then routes to the Studio.

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