feat(projects): add additional agent instructions

[incognitojam]

Note

Adds a project-level Additional instructions preference and applies it whenever T3 Code starts an agent session.

Problem

Projects cannot currently provide durable guidance to every coding-agent session, so users must repeat the same instructions in each thread.

What changed

• Add an Additional instructions textarea to Project Settings, shared across grouped checkouts.
• Persist the preference through orchestration events, projections, snapshots, and a fork migration.
• Pass instructions through native channels for Codex, Claude, Grok, and OpenCode.
• Prefix the first prompt of fresh Cursor sessions because Cursor ACP does not expose a native system-instruction channel.
• Retain instructions when provider sessions are recovered.
• Document the behavior and reset flow.

Verification

• Focused orchestration, projection, migration, recovery, and provider adapter tests pass.
• Contracts, server, and web typechecks pass.
• Targeted lint and formatting checks pass.
• Verified in an isolated T3 preview that Project Settings renders the new textarea and explanatory copy. Before/after capture is omitted because the shared preview screenshot endpoint repeatedly timed out.

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Written by an agent (T3 Code, gpt-5.6-sol).