feat(opencode): add per-session budget limit

[HHrddtu]

Adds an optional per-session spending budget that stops the assistant once the session cost reaches the limit.

• New budget field on sessions: schema, storage column, and DB migration (20260812223059_session_budget)
• PATCH /session/:id accepts budget (null clears it); sessions can be created with a budget
• The prompt loop stops after the step that crosses the budget and appends an explanatory text part; re-prompting does not start a new turn until the budget is raised
• TUI sidebar context panel shows spent/budget, adjusts the budget with mouse scroll (0.25 steps, scroll below zero clears it) and edits it by clicking and typing (Enter saves, Esc cancels)
• Regenerated SDK types with a nullable budget on the session update request

Tests: added/updated processor-effect, prompt, and schema-decoding tests.

[github-actions]

This PR doesn't fully meet our contributing guidelines and PR template.

What needs to be fixed:

• PR description is missing required template sections. Please use the PR template.

Please edit this PR description to address the above within 2 hours, or it will be automatically closed.

If you believe this was flagged incorrectly, please let a maintainer know.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential duplicate PRs found:

1. PR feat(opencode): add per-session budget limit #42202 - feat(opencode): add per-session budget limit

   • feat(opencode): add per-session budget limit #42202
   • Why: Same title and feature as the current PR feat(opencode): add per-session budget limit #42823. This appears to be an earlier version or duplicate of the exact same work.

2. PR feat(app): add voice input and session budget UI #42824 - feat(app): add voice input and session budget UI

   • feat(app): add voice input and session budget UI #42824
   • Why: Related to session budget functionality, specifically the UI component that displays and allows editing of the budget in the TUI sidebar, which is mentioned in the current PR's description.

Recommend reviewing PR #42202 first as it has an identical title and scope to PR #42823.

[github-actions]

This pull request has been automatically closed because it was not updated to meet our contributing guidelines within the 2-hour window.

Feel free to open a new pull request that follows our guidelines.