refactor: unify planning skills into /to-spec + /to-tickets

Executes the `unify-to-tickets` decision (personal-wiki #23), reworking
this PR from "add /to-plan" into the full planning-skills unification.

- Rename /to-prd → /to-spec. "spec" is now the single through-line term
  (the skill still opens with "you may know this document as a PRD" for
  discoverability).
- Merge /to-plan + /to-issues into one /to-tickets skill; delete
  /to-issues. /to-tickets emits a set of tickets, each declaring its
  blocking edges — one medium-agnostic artifact that reads as a
  sequential plan (local `tickets.md`, edges as text) or a parallelizable
  DAG (real tracker, native blocking links). The edges live in the ticket
  either way; the medium only decides whether anything acts on them in
  parallel.
- ask-matt now routes idea → /to-spec → /to-tickets → /implement.
- Sweep every cross-reference (docs pages, chain diagrams, CONTEXT.md,
  ADRs, .agents, setup templates) to the new names and vocabulary.
- Rewrite the changeset to describe the unification.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/productivity/handoff.md

@@ -14,7 +14,7 @@ npx skills update handoff
 
 `handoff` compacts the current conversation into a **handoff document** — a single write-up a fresh agent can read to pick up the work where you left off.
 
-It does **not** re-state what already lives elsewhere. Anything captured in a PRD, plan, ADR, issue, commit, or diff is referenced by path or URL, never copied. The document carries only the live thread — what you were doing, why, and what's next — and it's saved to your OS's temporary directory, not into the workspace, so it never becomes another artifact to maintain.
+It does **not** re-state what already lives elsewhere. Anything captured in a spec, plan, ADR, issue, commit, or diff is referenced by path or URL, never copied. The document carries only the live thread — what you were doing, why, and what's next — and it's saved to your OS's temporary directory, not into the workspace, so it never becomes another artifact to maintain.
 
 ## When to reach for it
 
@@ -26,11 +26,11 @@ Reach for this when a conversation has gone long enough that its context is at r
 
 - **The live thread** — what's in flight and why, in the conversation's own terms, minus anything already written down elsewhere.
 - **Suggested skills** — a pointer to the skills the next agent should reach for to continue.
-- **References, not copies** — links and paths to the PRDs, plans, ADRs, issues, and diffs that hold the settled detail.
+- **References, not copies** — links and paths to the specs, plans, ADRs, issues, and diffs that hold the settled detail.
 - **Redacted secrets** — API keys, passwords, and PII stripped before the document is written.
 
 The idea to hold onto is **compaction**: a handoff is the conversation squeezed down to just its resumable core, so a fresh agent inherits the momentum, not the noise.
 
 ## Where it fits
 
-`handoff` is a reach-for-it-anytime standalone — it sits at the seam between two sessions rather than inside a build chain. It pairs naturally with the artifact-producing skills whose output it points at: [to-prd](https://aihero.dev/skills-to-prd), because a finished PRD is exactly the kind of settled detail a handoff references instead of repeating. When you're unsure which skill fits the moment, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.
+`handoff` is a reach-for-it-anytime standalone — it sits at the seam between two sessions rather than inside a build chain. It pairs naturally with the artifact-producing skills whose output it points at: [to-spec](https://aihero.dev/skills-to-spec), because a finished spec is exactly the kind of settled detail a handoff references instead of repeating. When you're unsure which skill fits the moment, [ask-matt](https://aihero.dev/skills-ask-matt) routes you.