[FEATURE]: add wait built-in tool with countdown in footer

Summary

Add a Core built-in wait tool that blocks for N seconds while publishing
Tool.Progress updates, and render a countdown in the TUI footer instead of
the generic spinner.

Why

Claude Code has a WAIT tool that blocks (in tool-call queue, async to the
agent loop) and shows countdown in footer — useful for:

• CI / deployment wait loops (wait N seconds then check status)
• Avoiding busy-poll bash: sleep 1; check
• First real consumer of the Tool.Progress event (currently defined but unused)

We already have all the infrastructure:

• Tool.Progress event schema (packages/schema/src/session-event.ts:331-340)
• TUI footer spinner (packages/tui/src/component/prompt/index.tsx:1515-1594)
• App reducer already handles session.tool.progress (packages/app/src/context/server-session-v2-reducer.ts:276)
• Tool.make + permission.assert pattern (see webfetch.ts)
• Effect.gen for time-based orchestration
• Async tool execution via FiberSet.run (packages/core/src/session/runner/llm.ts:278) — wait runs in parallel with sibling tool calls in the same turn, like all other tools

Proposed contract

Input:  { seconds: 1..600, reason?: string }
Output: { waited: number, reason?: string }

Hard cap 600s to discourage wait(seconds=86400) (Claude Code removed wait
after abuse — same model here).

Implementation sketch

1. packages/core/src/tool/wait.ts — new built-in
   • Effect.gen with 1Hz ticker publishing Tool.Progress with { remainingSeconds, totalSeconds, reason } in structured/metadata
   • permission.assert({ action: "wait", resources: [] }) — every call requires approval unless allowlisted (same as webfetch)
   • Built-in registered in packages/core/src/tool/builtins.ts + new location node
2. packages/tui/src/component/prompt/index.tsx — match progress.structured.type === "wait" → ⠋ 12s remaining (reason); fallback to generic spinner for non-wait tools
3. packages/app/src/... — RunningToolPart (or wherever running tool is rendered) reads metadata; if structured.type === "wait" show 12s remaining. App reducer already passes metadata through (packages/app/src/context/server-session-v2-reducer.ts:282).
4. Tests: extend packages/core/test/session-tool-progress.test.ts with wait-specific cases (cap enforcement, permission gating, progress event cadence)

Out of scope (future)

• Stage 3: URL polling source (wait({ url, expect: 2xx }))
• Stage 4: pluggable sources via opencode.tools.register

Open questions (to resolve during impl)

1. How exactly to publish Tool.Progress from inside Tool.make execute — the publisher lives in runner/publish-llm-event.ts; need to verify the channel exposed to tool leaves. May need a new lightweight ctx.progress.emit(structured) accessor.
2. Permission resource for wait (webfetch uses URL; wait has no resource — []?).
3. Footer vs timeline rendering trade-off (TUI has both — pick one or both).