Add agent 'stuck' tool for explicit stuck-state reporting

[Cheggin]

Problem

Today, detecting when an agent is stuck (e.g., looping on a login wall, blocked on a captcha, waiting for a 2FA code, retrying the same failing action) is implicit and unreliable. We rely on heuristics over the trajectory rather than the agent telling us directly. This produces:

• Late detection — the agent burns steps/tokens before we surface the issue
• Ambiguous UX — the user sees activity but no clear signal that progress has stopped
• Hard-to-diagnose failures — no first-class reason string explaining why the agent thinks it's stuck

Proposal

Add a new tool to the Browser Harness (and expose it through the agent loop) that lets the agent explicitly emit a stuck state with a free-form message.

stuck(reason: string, details?: string)

Example calls the agent could make:

• stuck("Hit a login page and I don't have credentials")
• stuck("Blocked on captcha — needs human")
• stuck("Site keeps redirecting back to home, can't reach checkout")
• stuck("Same selector failed 3 times in a row")

When invoked, the harness should:

1. Mark the session as stuck (new state, distinct from running / done / error)
2. Forward the reason string to the AgentPane so the user sees a clear overlay/banner: "Agent is stuck: {reason}"
3. Surface a Resume / Provide Input / Stop action so the user can unblock the agent
4. Emit a telemetry event so we can analyze stuck patterns over time

Open questions

• Should stuck halt the run, or just signal and let the agent keep trying? (Lean: halt and wait for user input, similar to a cancel-with-resume.)
• Do we want a structured category enum (auth, captcha, navigation, selector, unknown) alongside the free-form reason for analytics?
• How does this interact with the existing AgentPane end-state overlay? Likely reuse the unified centered overlay with a sub-line for the stuck reason.

Out of scope

• Auto-detection heuristics — this issue is about giving the agent a way to tell us it's stuck. Heuristic detection is a separate follow-up.

Acceptance criteria

• New stuck tool registered in the harness tool set
• Session state machine has a stuck state with reason payload
• AgentPane renders the stuck reason in the unified overlay with Resume/Stop actions
• Telemetry event emitted on stuck
• Docs updated for the new tool