feat(ship): skip version-bump/CHANGELOG/title-rewrite when the repo has no VERSION file

[merit-blake]

feat(ship): skip version-bump/CHANGELOG/title-rewrite when the repo has no VERSION file

Repos that stamp versions at release/queue time (merge-queue driven repos,
release-please/tag-based flows) have no VERSION file, but /ship today
manufactures one: gstack-version-bump treats a missing file as 0.0.0.0,
classifies FRESH, and write creates VERSION at the bumped slot - then
writes a CHANGELOG release heading and rewrites the PR title to vX.Y.Z.W.
On a queue-managed repo every such PR textually collides with every other
(we hit three pairs of open PRs claiming identical versions in one repo
before tracing it here).

The convention was already keyed off the VERSION file's presence; this
makes the absence a first-class state instead of a 0.0.0.0 default:

• gstack-version-bump classify: new NO_VERSION_FILE state when the
  resolved version file does not exist (never invents 0.0.0.0, never
  creates the file; also covers a branch that deletes VERSION to retire
  the convention). write/repair are unchanged.
• ship Step 12: NO_VERSION_FILE dispatch arm - skip Steps 12-13 and the
  PR/MR title version invariant; plain :
  titles; never
  create a VERSION file. Idempotency note updated for re-runs.
• ship Step 13 (CHANGELOG): explicit skip under NO_VERSION_FILE.
• ship title invariant: now conditional on Step 12 having produced a
  version.
• land-and-deploy Step 3.4: VERSION drift detection prints n/a and skips
  when neither the head nor the base has a VERSION file.
• Tests: NO_VERSION_FILE for a never-versioned repo (asserts classify
  stays a pure reader and creates nothing) and for a branch that deleted
  VERSION while base still has it. bun test on gstack-version-bump,
  ship-version-sync, gstack-next-version: 59 pass / 0 fail.

Generated SKILL.md files regenerated with bun run gen:skill-docs.

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