Welcome providers: Vespyr, Fastfetch, Neofetch and None #135

Goal

Let users choose what appears at the top of a new presentation session.

UX / product direction

• The selection UI mirrors /prompt provider selection: list/gallery, install/detection state, preview, persisted choice, safe fallback. Built on Provider framework and unified provider UX #134.
• A /welcome gallery command is optional; include it only if natural. The /settings provider page is enough for v1.

Behavior

• Welcome content is session presentation, not command output. It never enters shell history, /copy, exit status, command counts or Smart Folding.
• New presentation sessions (launch, /clear) use the selected welcome.
• Resumed archived sessions keep the welcome that belonged to them, stored as a session snapshot like today's WelcomeSnapshot. Changing provider never rewrites history.
• Fastfetch/Neofetch run the user's installed tool with the user's own configuration. NMSh does not reimplement them. Output is captured once at session start (argv, timeout, bounded size, no stdin) and stored as the snapshot.
• Narrow terminals clip or hide the welcome rather than wrapping garbage.
• Failure or timeout falls back to Vespyr with a quiet notice.
• Real images can come later through terminal image protocols (Rich terminal preview surfaces #151). v1 uses text logos; fetch-tool image logo modes are not passed through blindly.

Non-goals

• Reimplementing fetch tools.
• Arbitrary custom welcome scripts.
• New animation beyond the existing blink.

Acceptance criteria

• All four providers are selectable, previewed and persisted.
• Fastfetch/Neofetch show installed/missing/version. Missing tools offer install via Optional shell-tool discovery and first-run setup #9 only with explicit confirmation.
• /copy, history, Smart Folding and exit status are unaffected.
• /resume restores the original welcome; /clear uses the current provider.
• A slow or hanging fetch tool never blocks the composer beyond the timeout.
• Physical Ghostty check including narrow widths.

Dependencies / related

• Part of Provider framework and unified provider UX #134; depends on its foundation checkpoint.
• Welcome rows render through Transcript presenter: centralize command/output row presentation #122. Related Fresh-session welcome header and NMSh cat mascot #56, Optional shell-tool discovery and first-run setup #9, Rich terminal preview surfaces #151.

Research references

• Fastfetch: https://github.com/fastfetch-cli/fastfetch