feat: add sub-agent cost breakdown with recursive session walker

[JudicaelPoumay]

Summary

Add sub-agent cost breakdown with recursive session walker
Add recursive costs() utility for sub-agent expenditure tracking

The cost of the session/agents now includes the cost of subagents.
This recursively applies when inspecting sub-agents.
Subagents cost is visible in the chat

Comprehensive test coverage (13 new tests)

• Fixes Cost of session doesn't include cost of subagents #11027
• Fixes [FEATURE]: Display total cost for session #4925

Visual changes

1. cost display unchanged, but now accounts for the total session (including sub-agents).
   When inside an agent, display the total cost of that agent and its sub-agents.

3. Added visible agent task cost inside the chat

Similar changes for the Web app

Verification

Passed packages/util/src/cost.test.ts (7 tests)
Passed packages/app/src/components/session/session-context-metrics.test.ts (6 tests)
Typecheck passes for all affected packages (@opencode-ai/ui, @opencode-ai/util, @opencode-ai/app, opencode)
Note: @opencode-ai/enterprise typecheck failure is pre-existing on dev (unrelated custom-elements.d.ts issue)

[github-actions]

Hey! Your PR title feat(util,app): add sub-agent cost breakdown with recursive session walker doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[alexyaroshuk]

This change seems unrelated. Is this related?

[JudicaelPoumay]

No, this is a draft PR. Mostly vibe coded so far, so there a lot of AI garbage still. Had to push to work on another computer.

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[github-actions]

Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.