[raiseCatError]

Draft, do not merge before v0.21 ships. #430 has carried dev into release/v0.21.0; a fix v0.21's physical QA needs would reach the release the same way, so anything merged into dev before the release would ride along. Part of v0.22 (#431); approved as decision D3.

Why (#366)

With $VISUAL and $EDITOR unset and no Zed or VS Code around NMSh, /open could only say that no editor was known, even when Neovim, Helix or Micro was installed.

What changed

• Settings → Sessions → Terminal editor opens the existing provider chooser (a new editor provider family, so /providers and Ask know it too). It lists Neovim, Helix, Micro, Vim, nano, Emacs (-nw) and Kakoune, marks which are installed here, and previews the exact command /open would compose for the highlighted one (nothing runs).
• Never assumed, never installed: detection is a PATH lookup only (no version run, so no editor executes); there are no install recipes (the upstream page is named instead). An editor that is not installed cannot be chosen: the chooser says why and saves nothing.
• Explicit preferences first (resolveHostActions): Open with Zed or VS Code, then the editor NMSh runs inside, then VISUAL/EDITOR, then the chosen editor (only when installed). If the chosen one is missing, /open says so plainly. With nothing chosen, the message names the editors installed here and where to choose.
• Each editor's own line and column form (editorArguments): Helix file:N:C, Micro +N:C, nano +N,C, Emacs +N:C; Vim, Neovim and Kakoune +N (no documented column flag). Unknown editors get no guessed flag.
• Explicit execution kept: a terminal editor's command goes into the composer for you to run, quoted for the session's shell, so a path with spaces or shell characters stays one argument. The real PTY, composer drafts, input ownership and focus restoration are unchanged; the native /edit is untouched.

Dependency: o in /view (#434) uses the same bridge; it picks up the chosen editor once both are merged. This PR is independent of #434.

Verification

• New tests: tests/terminalEditor.test.ts (precedence: environment, surrounding editor, explicit Open with, chosen, missing; per-editor forms; adversarial path; configuration normalization; the family; the Settings row; no recipes or version runs in the catalog) and tests/terminalEditorApp.test.ts (the real TerminalApp with stub executables on a controlled PATH, VISUAL/EDITOR/ZED_TERM cleared: the chooser detects installed and missing editors, previews the command, refuses a missing one, saves an installed one, and /open composes Helix's form quoted). The new module and setting do not exist on dev.
• Suites: host actions, viewer app, providers overview, Ask (including the coverage audit, which now knows the family), column gutter and Settings: all pass. Full canonical npm run verify on 4ab9f44a: 3141 pass, 5 fail, 2 skipped.
  • 3 were a gap in this PR, fixed in 898fbc33: Setup's coverage audit requires every Settings row and panel to be reachable from Setup, and the new row was not. Setup → Sessions & alerts → Terminal editor now opens the same chooser over Setup and comes back to the same step with the choice in the draft (other unapplied edits kept). New test in tests/terminalEditorApp.test.ts, which fails without the fix; Setup coverage, Setup order and the editor tests: 21/21.
  • 2 were live tests timing out under load (compatibility harness, live zsh input ownership): 16/16 when rerun at load 5.
  • A fresh full run on the fixed head is recorded below when done.
• CI runs on this PR.

Physical QA (not done)

Ghostty and Terminal.app with VISUAL/EDITOR unset (for example env -u VISUAL -u EDITOR nmsh): /open README.md:3 explains and names the installed editors; choose one in Settings → Terminal editor; /open README.md:3:2 puts its command in the composer at the right line; Enter opens it; quitting returns to NMSh. Choosing an editor that is not installed is refused. Inside Zed's and VS Code's integrated terminals the surrounding editor still wins.

Refs #366