Fix onboard intro capture and Git instructions routing

[0xroylee]

What Changed

• Added a top-level /git page for the Git instruction editor and wired the workspace navigation to it.
• Restored the Git instruction panel as a dedicated component backed by the existing GitHub settings API.
• Updated the shell navigation tests to cover the new /git route contract.

Why

• The old Git settings page lived under /settings/git, but the current shell now surfaces Git as its own operator page.
• This keeps commit and pull request instruction editing reachable from the main navigation without relying on the removed settings route.

How to Test

• bun test packages/web/tests/web-shell-navigation.test.ts packages/web/tests/settings-navigation.test.ts
• bun test
• Open http://127.0.0.1:3000/git and confirm the Commit and Pull request fields render with the saved values.

Additional Notes

• bun run --filter web build passed and confirmed the /git route is included in the app build.
• A repo-wide bun run check still reports unrelated pre-existing formatting issues in chat-room files outside this change.