Loki Mode

UX: less friction, more wow

Six experience improvements that cut the number of commands and flags you need to
learn, make output worth screenshotting, and tell you the next step everywhere.
All additive and opt-out; default behavior is unchanged.

• One-command forward motion: loki next reads the build's own state and runs the
  right next step (resume / ship / why), confirm-then-run on a TTY, print-only
  with --dry-run or off a TTY. loki ship finishes a build in one command:
  optional preview, the same diff quality gates as review, and the exact
  branch-aware PR command on a clean result (print-only; never pushes).
• Zero-config loki start: when run with no spec it auto-discovers an on-disk
  spec (prd.md, a single top-level .md/.yaml), shows a resume hint if a previous
  build here was interrupted (naming the last checkpoint), and prints a one-line
  pre-flight summary. All opt-out; nothing changes when a spec is given or none
  is found.
• Visible delight: an inline completion card after a build, a "Building:" headline
  on start, app-live / last-run lines in status, and a polished COMPLETION.txt
  receipt. Display-only and TTY-gated (byte-identical off-TTY and under --json).
• Actionable guidance: the PRD-not-found messages are now unified and tell you
  exactly what to do; the instance-running path explains recovery; tab completion
  is spec-aware.
• Discoverability + first-run wow: loki doctor ends with a copy-paste next step,
  a one-command completions install, quickstart surfaced on the landing/welcome
  screen, and a drift-proof completion-coverage check so the command list cannot
  silently fall out of sync again.

Full suite green (1422 pytest + bun + shell suites); completion-coverage drift
check passing.