feat(clients): reload provider sessions per thread without restarting T3 Code

[DavidIlie]

What changed

Provider processes load MCP servers and runtime configuration when they start. Existing threads therefore required a full T3 Code restart to pick up changes such as enabling Computer Use.

This adds Reload agent session to thread actions and the command palette on web/desktop, plus thread controls on mobile. It stops only that thread's provider process; the next message starts a new process and resumes the stored provider conversation with current configuration. Other threads and the environment keep running.

Reload reuses the existing stop/resume lifecycle and works across Codex, Claude Code, Cursor, Grok, and OpenCode. An idle-only precondition is checked atomically by the decider, so a stale client cannot interrupt a turn started elsewhere.

Closes #5437.

Screenshots

Before	After

Validation

• 107 focused client, orchestration, reactor, and provider tests passed
• Contracts, client-runtime, web, mobile, and server typechecks passed
• Targeted lint, formatting, and git diff --check
• Isolated web pass confirmed reload followed by a successful resumed turn through a fresh provider process

Model: GPT-5.6 Sol · Harness: Codex in T3 Code

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new user-facing feature (reload agent session) with UI controls across mobile and web, backend decider logic changes, and contract modifications. New features introducing user-facing behavior warrant human review.

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