feat(product): complete retained multi-session terminal

[drewstone]

Outcome

Braid now presents concurrent retained agent work as one terminal product.

The branch implements start, stream, focus switching, targeted cancellation, native continuation, conversation branching, workspace fork, typed interactions, trace analysis, supervisor controls, and parallel sessions through shared Runtime and Agent Interface contracts.

All 51 exported terminal components map to 25 component documents.
Each document states the smallest general implementation that satisfies its contract.

Product proof

• Core tests: 860 passed, 2 intentional skips, 0 failed.
• Performance checks: 16 of 16 passed.
• Native SQLite checks: 2 of 2 passed at 10,000 and 100,000 events.
• Release-evidence checks: 32 of 32 passed.
• Formatting and lint: 803 files checked with no findings.
• Real Tangle multirun: two concurrent conversations streamed together, focus switched both ways, only the selected run was cancelled, replay stayed duplicate-free, and cleanup returned to zero owned resources.
• Real Pi interaction: permission paused the runner, crossed CLI Bridge, resumed once, and rejected a stale duplicate.
• Interaction and restart review: 21 of 21 focused cases passed.

Visual proof

Concurrent work at 120 by 40

Real keyboard flow at 80 by 24

The branch also contains verified captures at 40 by 12, 80 by 24, 120 by 40, and 200 by 60.

Release blockers

This PR stays draft until each item below passes against the exact release cohort.

• Sandbox exact interactive control and durable fork metadata: agent-dev-container PR 6392.
• Tangle provider release against the published Sandbox SDK.
• Confidential x86_64 SSH and revoke path: ai-agent-sandbox-blueprint PR 152 and its production rollout.
• LIVE-08 retained interaction, LIVE-09 workspace fork, and LIVE-10 confidential placement.
• The strict three-runner bridge check after the OpenCode quota reset.
• Final package preparation, registry installation, and post-publication smoke checks.

No fixture, fake provider, or unavailable row is counted as live proof.