An admin renames or recolours a label from the terminal's Labels screen with `e` #8017

On the terminal's Labels screen, e on the selected label now opens the same form n uses, with the name and colour prefilled as name hex. I ran bun x tsc --noEmit, scripts/gate.ts, test/tui-labels.test.ts and test/mcp-size.test.ts, and all passed. I did not run the full suite; the forge's check does that on the push.

• Saving: the form calls updateLabel and reloads the list in name order. A leading # on the colour is accepted, and the colour is lowercased.
• Refusals: a taken name or a bad colour shows updateLabel's words on the form and keeps what was typed. If the label was deleted underneath, the form says LABEL_WORDS.gone. If the user was demoted mid-edit, it says "Only a repository admin can make a label." (the existing NOT_ALLOWED_LINE, which now covers edits too).
• Who gets e: it is listed and bound only for a repository admin, as n and d are. A reader gets no e and the key does nothing.
• Form helper: I reused promptScreen and the name-and-colour parsing that n already has, and added no second form. The parsing is now a shared parse, and the refusal wording a shared refusal.
• Tests: test/tui-labels.test.ts gains three. They cover an admin renaming and recolouring a label that an issue carries, a taken name refused in place, and a reader with no e. The first checks the form is prefilled, that the label keeps its id, and that the issue still shows it under the new name. I did not run them against the old code to watch them fail.
• MCP: nothing touched.

To try it, open a repository you administer, go to Labels, move to a label and press e.

Closes #7868

On the terminal's Labels screen, e on the selected label now opens the same form n uses, with the name and colour prefilled as name hex. I ran bun x tsc --noEmit, scripts/gate.ts, test/tui-labels.test.ts and test/mcp-size.test.ts, and all passed. I did not run the full suite; the forge's check does that on the push.

• Saving: the form calls updateLabel and reloads the list in name order. A leading # on the colour is accepted, and the colour is lowercased.
• Refusals: a taken name or a bad colour shows updateLabel's words on the form and keeps what was typed. If the label was deleted underneath, the form says LABEL_WORDS.gone. If the user was demoted mid-edit, it says "Only a repository admin can make a label." (the existing NOT_ALLOWED_LINE, which now covers edits too).
• Who gets e: it is listed and bound only for a repository admin, as n and d are. A reader gets no e and the key does nothing.
• Form helper: I reused promptScreen and the name-and-colour parsing that n already has, and added no second form. The parsing is now a shared parse, and the refusal wording a shared refusal.
• Tests: test/tui-labels.test.ts gains three. They cover an admin renaming and recolouring a label that an issue carries, a taken name refused in place, and a reader with no e. The first checks the form is prefilled, that the label keeps its id, and that the issue still shows it under the new name. I did not run them against the old code to watch them fail.
• MCP: nothing touched.

To try it, open a repository you administer, go to Labels, move to a label and press e.

I pushed the fixes for both points, except that I dropped the state.clamp after saving instead of testing it. tsc, scripts/gate.ts and test/tui-labels.test.ts pass.

• New tests: I added two tests for the gone and demoted paths.

- Deleted underneath: the label is deleted while the form is open, then the form is submitted. It shows "That label no longer exists." and the list stays empty. - Demoted: the actor is demoted mid-edit, then the form is submitted. It shows NOT_ALLOWED_LINE, and the label is unchanged. - Mutation check: I checked the first test fails when the not_found && admin() branch is removed. I did not run that check for the demoted test.

• Wording: NOT_ALLOWED_LINE now reads "Only a repository admin can change a label." That fits both creating and editing, so there is no per-action line.
• Docs: there is no terminal section in the README that describes n and d. They are described in docs/APPS.md, so that is where I added e. It now says e opens the same form prefilled, a refusal keeps what was typed, and a reader is offered none of n, e or d.
• The clamp: a rename never changes how many labels there are, so the state.clamp after saving could never do anything, and no test could make it fail. I removed it rather than write a test that could not fail without it.