chore: merge upstream v1.4.0 into dev

[jo-s-eph]

Issue for this PR

Closes #none

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR merges the latest upstream v1.4.0 release into our dev branch.
During the merge, several structural changes from upstream conflicted with our custom Mammouth Code setup. I resolved these by:

1. Re-applying our custom Mammouth branding across the TUI and UI components.
2. Preserving our custom mammouth-ai provider logic in src/provider/models.ts and src/cli/cmd/tui/routes/session/index.tsx.
3. Ensuring our custom MAMMOUTH_* environment variables and flags (in flag.ts) override the default upstream OPENCODE_* equivalents.
4. Disabling newly added upstream GitHub actions that don't apply to our fork.
5. Fixing a few TypeScript errors (like the model variable redeclaration and missing Bun.sleep imports) that popped up as a result of the conflict resolutions.
   The result is a clean merge that brings in 1.4.0 improvements while fully maintaining our custom Mammouth ecosystem.

How did you verify your code works?

• Ran bun install to successfully rebuild bun.lock.
• Ran bun turbo typecheck across all packages, which now passes completely without errors.
• Executed the local test suite via bun test to ensure core functionality hasn't broken.

Screenshots / recordings

N/A - Backend and upstream merge sync

Checklist

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

[github-actions]

Hey! Your PR title Upstream merge 1.4.0 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.

[github-actions]

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

What needs to be fixed:

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[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.