feat(opencode): add per-session budget limit

[HHrddtu]

Issue for this PR

No linked issue — new feature.

Type of change

• New feature

What does this PR do?

Adds an optional per-session budget that stops the assistant when the session cost reaches it, plus a small widget in the TUI sidebar Context panel to view and set it.

• budget field on the session schema (v1 + current), a nullable budget column on the session table, and a generated drizzle migration
• PATCH /session/{id} accepts budget (a number sets it, null clears it); sessions can be created with a budget
• Enforcement in the V1 prompt loop: after each step-finish the accumulated cost is compared against the budget; when crossed, a text part "Session budget reached. Increase the budget to continue." is appended and the loop stops. Re-prompting does not start a new provider turn until the budget is raised (guard at loop entry)
• TUI: the Context panel shows $0.42 / $2.00 budget (red when exceeded, unlimited budget when unset). Mouse scroll over the row adjusts it in $0.25 steps (scrolling below $0.25 clears it), clicking switches to an input — Enter saves, Esc cancels
• SDK types regenerated; the OpenAPI encoder drops the null arm of Schema.NullOr wrapped in Schema.optional, so the sdk build script now widens the update-side budget types to number | null

Known limitation: enforcement lives in the V1 prompt loop. The durable V2 runner does not compute cost yet (cost: 0 is hardcoded in Step.Ended), so budget stopping is a no-op there until V2 cost accounting lands.

Small extra: packages/core/script/migration.ts got a Windows fix — generatedMigrations split on / only, which broke migration generation on Windows. It was required to generate the migration for this PR.

How did you verify your code works?

• bun typecheck in packages/schema, core, opencode, tui, sdk/js — all clean
• bun test test/session in packages/opencode: 383 pass (new tests: budget stops the turn, exhausted budget exits the loop without an LLM request, budget round-trips through Session.Info)
• new packages/tui/test/feature-plugins/sidebar-context.test.tsx: headless widget tests (render, wheel up/down, clear, click-to-edit, Enter save, Esc cancel) using the OpenTUI test renderer with mock mouse/keys
• migration generator --check passes
• manually ran bun dev and exercised the widget with mouse and keyboard

Screenshots / recordings

Char frame from the headless test renderer (the Context panel of the session sidebar):

Context
0 tokens
0% used
$0.42 spent
$0.42 / $2.00 budget

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[github-actions]

Automated PR Cleanup

Thank you for contributing to opencode.

Due to the high volume of PRs from users and AI agents, we periodically close older PRs using automated criteria so maintainers can focus review time on the most active and community-supported contributions.

This PR was closed because it matched the following cleanup criteria:

• The PR was created more than 1 month ago
• The PR had fewer than 2 positive reactions
• Positive reactions are counted as thumbs-up, heart, celebration, or rocket reactions on the PR

PRs created within the last month are not affected by this cleanup.

If you believe this PR was closed incorrectly, or if you are still actively working on it, please leave a comment explaining why it should be reopened. A maintainer can review and reopen it if appropriate.

Thanks again for taking the time to contribute.