An admin changes the default branch from the terminal's About card with `b`, picking from the repository's branches #8302

An admin can now change the default branch from the terminal's About card with b. The new test passes, along with the other tui-about-* tests and mcp-size. The gate and tsc printed nothing. I did not run design-lint, since nothing under src/web/ changed.

• Who gets it: b "default branch" shows in the card's keys only when may() is true. A press from a non-admin does nothing.
• Picking: b reads the repository's branches and opens the picker, with the current default marked "default". Choosing the current default does nothing.
• Confirming: a different branch asks make <branch> the default? y/n. Cancel is selected first, so enter alone changes nothing. y calls updateRepoSettings(app, actor, repo, { defaultBranch }), which moves git HEAD first and is audited as from Settings. n or esc leaves the card as it was.
• Refusals: an unknown_branch refusal shows "<branch> is no longer a branch here" as a toast and changes nothing.
• The row: the "default branch" row now reads the card's current repo before the facts it was opened with. Without that it would have kept showing the old branch after saving.
• Test: test/tui-about-default-branch.test.ts, on a repo with main, trunk and gone, covers four cases:

- Admin picks trunk and presses y: HEAD and the row move. - n and esc: neither HEAD nor the row changes. - A branch deleted while the confirm is open: toast, nothing changes. - A writer who is not an admin: no b, and a press changes nothing.

• Left out: test/tui-about-golden.test.ts is unchanged, since it passed as it was. There is no way to type a branch name, and I did not touch the MCP, docs/MCP.md, README.md or src/http/server.ts.

The only source change is src/tui/screens/about.ts.

Closes #8067

An admin can now change the default branch from the terminal's About card with b. The new test passes, along with the other tui-about-* tests and mcp-size. The gate and tsc printed nothing. I did not run design-lint, since nothing under src/web/ changed.

• Who gets it: b "default branch" shows in the card's keys only when may() is true. A press from a non-admin does nothing.
• Picking: b reads the repository's branches and opens the picker, with the current default marked "default". Choosing the current default does nothing.
• Confirming: a different branch asks make <branch> the default? y/n. Cancel is selected first, so enter alone changes nothing. y calls updateRepoSettings(app, actor, repo, { defaultBranch }), which moves git HEAD first and is audited as from Settings. n or esc leaves the card as it was.
• Refusals: an unknown_branch refusal shows "<branch> is no longer a branch here" as a toast and changes nothing.
• The row: the "default branch" row now reads the card's current repo before the facts it was opened with. Without that it would have kept showing the old branch after saving.
• Test: test/tui-about-default-branch.test.ts, on a repo with main, trunk and gone, covers four cases:

- Admin picks trunk and presses y: HEAD and the row move. - n and esc: neither HEAD nor the row changes. - A branch deleted while the confirm is open: toast, nothing changes. - A writer who is not an admin: no b, and a press changes nothing.

• Left out: test/tui-about-golden.test.ts is unchanged, since it passed as it was. There is no way to type a branch name, and I did not touch the MCP, docs/MCP.md, README.md or src/http/server.ts.

The only source change is src/tui/screens/about.ts.

I made all of juniper's requested fixes and pushed them to the same branch. The tui-about-* tests pass (24), and tsc and the gate printed nothing.

• Agent admins: b is now gated on mayPerson(), like V and w, so an agent admin is no longer offered it. The test now has a writer who is not an admin and an agent admin who owns a repo. Neither is offered b, and pressing it does nothing.
• README: I added a "Default branch at the terminal" bullet next to the V and w ones. It covers:

- who is offered b; - the picker; - the y/n confirm with Cancel first; - the save through updateRepoSettings, audited as Settings; - the unknown_branch toast.

• Test tidy:

- I merged the duplicate actions/branches.ts import and removed the whitespace-only line. - The success case now asserts the toast reads trunk is the default branch. - It also reopens the picker and checks that the default hint is on trunk and no longer on main.

• Scope: I kept the change small and single-purpose, as asked.