Provider ask UX drift: unify on '<provider> ask' subcommand, deprecate --vercel/--flyio/--railway/--verda flags

[rafeegnash]

Problem

Provider routing on the CLI is inconsistent:

• Dedicated subcommand: clanker sentry ask "...", clanker linear ask "...", clanker notion ask "..."
• Flag on root ask: clanker ask --vercel "...", clanker ask --flyio "...", clanker ask --railway "...", clanker ask --verda "..."
• Both: Cloudflare, K8s

Users discovering the CLI will assume one convention and hit "flag does not exist" errors. The dedicated-subcommand form is the better one (richer flags, per-provider history).

Where

• cmd/root.go:117-133 — provider registration
• cmd/ask.go:1449-1488 — --vercel/--flyio/--railway/--verda branches
• cmd/vercel.go, cmd/flyio.go, cmd/railway.go, cmd/verda.go — missing ask subcommand

Fix

Pick the dedicated-subcommand shape. For each of vercel, flyio, railway, verda:

1. Add an ask subcommand mirroring cmd/sentry.go's shape (load history, gather context, call AI, save history)
2. Keep the --vercel/--flyio/--railway/--verda flags as hidden aliases for one minor version so existing users aren't broken
3. Deprecate in changelog
4. Remove in a follow-up release

Acceptance criteria

• clanker vercel ask "..." works identically to today's clanker ask --vercel "..."
• clanker --help shows the dedicated subcommands consistently
• Old --vercel flag still works but logs a deprecation warning to stderr
• After the deprecation cycle, the old flags are removed