feat: open external files from browser via "i" key

[oobagi]

Problem

Opening external files (e.g. ~/.zshrc, config files) currently requires exiting
the TUI and running notebook ~/.zshrc from the CLI. This breaks flow — the whole
point of the TUI is staying in it.

Context

The plumbing already exists:

• openFile() in cmd/file.go handles reading, editing, saving, and permission
  preservation for arbitrary files
• recents.RecordExternal() tracks external files in the Recent section after opening
• Input mode (startCreate, startRename in browser.go) provides a status-bar
  text prompt pattern with Enter/Esc handling and cursor blink
• External files already render in recents via format.ShortenHome() and can be
  re-opened from there

The new feature would wire these together: i → input prompt → validate path → open file.

Possible approaches

Approach A: Reuse inputMode directly

• How: Add startImport() following the startCreate()/startRename() pattern.
  Set inputPrompt = "Open file:", resolve path in inputAction callback, then
  set m.selected with FilePath and quit to the browse loop.
• Pros: Minimal new code, fully consistent with existing UX
• Cons: No tab-completion for paths (can be added later)
• Files: internal/browser/browser.go (key handler + startImport), help overlay

Approach B: Selection-based with path in Selection

• How: Same as A but instead of opening inline, return a Selection{FilePath: path}
  so the browse.go loop handles opening via existing openFile() path
• Pros: Keeps editor launch in one place (browse.go), external files return to L0
  correctly via existing FromRecent logic
• Cons: None significant — this is the cleaner version of A
• Files: internal/browser/browser.go, cmd/browse.go (already handles FilePath)

Approach B is recommended — it reuses the browse loop's existing external file handling.

Tasks

• Add startImport() method using the inputMode pattern
• Bind i key at L0 (confirm unbound — currently unused)
• In the input action: expand ~, resolve to absolute path, validate file exists
  and is readable, return Selection{FilePath: path, FromRecent: true}
• Add i Open file to browser help overlay
• Add test for import flow (input mode → path → selection)

Test plan

• Press i at L0 → status bar shows "Open file:" prompt
• Enter valid path (absolute, relative, ~/...) → file opens in editor
• Save and quit editor → file appears in Recent section
• Enter nonexistent path → inline error, stays in browser
• Enter unreadable path → inline error with OS message
• Esc during prompt → cancels cleanly, no side effects
• Existing external file re-opening from recents still works

Scope

Type: feature
Size: small