v0.7.0 — board fidelity (cascade + letter phases)

[0.7.0] — 2026-06-22 — Board fidelity: merged-state cascade + letter phases

A high-severity board-fidelity fix (spec 013): merged specs no longer strand
their task-phase sub-issues in Todo, and letter-indexed phases no longer produce
zero sub-issues. Recommended upgrade for anyone with merged specs. Additive;
extension.id stays linear; no command/hook change; numeric specs are
byte-identical.

Lifecycle cascade to task-phase sub-issues — spec 013

• The board stops lying about merged work. When a spec reaches
  ready_to_merge / merged, every one of its task-phase sub-issues is driven to
  Done — overriding the tasks.md checkbox ratio — so a merged spec's phases
  read Done without hand-ticking every box. Previously a sub-issue's state came
  only from the checkbox ratio, so merged specs stranded their phases in Todo.
  Non-terminal specs are unchanged (state still tracks the checkbox ratio). The
  heal is one-time on the next reconcile, then zero-churn. (Amends FR-005/FR-013.)
• Letter-indexed phases now create sub-issues. The ## Phase N grammar
  accepts a single-letter index too (## Phase A — …, mapped A→1 … Z→26), so
  specs that index phases with letters no longer silently produce zero
  sub-issues. The label stays task-phase:<ordinal> and the title renders
  Phase 1 — …; numeric specs are byte-identical. Genuinely malformed headers
  (## Phase one, ## Phase 1Setup) still raise the near-miss warning.
• Sub-issue descriptions (the tasks.md checklist mirror) are unchanged —
  only the workflow state cascades. extension.id stays linear; no command or
  hook change. (A parity follow-up applies the same cascade to the spec-kit-jira
  sibling.)