A branch's sentences live in one module, read by the branches page, MCP, the mirror and the terminal #5713

Replaces #5651 with the same done, started fresh from main. Its first try went red on the unrelated pull-page separators test (now its own issue), and its second on the check machine itself.

For: Making or deleting a branch reads the same from every door (page, API, MCP, terminal) because it is said from one place. Today each sentence below is typed out separately in src/web/pages/branches.ts, src/http/mcp.ts, src/actions/branches.ts, src/origins/git-mirror.ts, src/tui/screens/refs.ts, src/tui/screens/workspace.ts.

Done:

• [ ] src/actions/branch_words.ts exports these sentences (new, or the module that already owns that concept; say which in the pull):

- "That branch was not created." - "That branch was not deleted." - "must not already exist" - "give the branch a name"

• [ ] Every file named imports them, and grep -rn for each sentence under src/ finds only the module.
• [ ] Pages, screens and answers are byte-for-byte unchanged: every existing test passes as it is.
• [ ] A new test/branch-words.test.ts asserts each file named no longer contains the literal (read as text), and fails if a copy is typed back.
• [ ] Before pushing, run test/branch-words.test.ts, the tests of the files you touched, bun x tsc --noEmit and, if src/web changed, bun scripts/design-lint.ts. A red test elsewhere that main also has is named in the pull, not a reason to stop.

Files: src/actions/branch_words.ts, src/web/pages/branches.ts, src/http/mcp.ts, src/actions/branches.ts, src/origins/git-mirror.ts, src/tui/screens/refs.ts, src/tui/screens/workspace.ts, test/branch-words.test.ts. Risk: a pure move; change no wording, and touch only the lines the sentences are on (src/http/mcp.ts and src/http/api.ts are edited in parallel).