v4.6.0 — one tracker per repo: local markdown by default, or GitHub/Jira only

With a tracker set, specs and tickets lived twice: .specs/ files plus issues that nothing updated. Now each repo has one home. Without setup, nothing changes: markdown in .specs/.

Added

• setup takes an argument: /hb:setup [local | github | jira <KEY>]. Rerun to switch; new work follows the new choice, nothing migrates
• skills/setup/issue-tracker-local.md: the local default as a seed, beside GitHub and Jira

Changed

• GitHub or Jira set → specs and tickets live only there. No .specs/specs/ or .specs/tickets/ files, no **Issue** lines
• Seeds are short plain-language rules: where things live, when a ticket is done, how it closes. GitHub uses native gh flags (--parent, --blocked-by, --json subIssues,blockedBy)
• to-spec, to-tickets, build, to-pr, review, triage: follow .specs/issue-tracker.md, else the local seed. No per-tracker branches in skills
• build: ticks boxes in the tracker and closes the ticket after its commit. GitHub on a feature branch leaves it for the PR
• to-pr: links open tickets referenced in the branch's commits (GitHub: Closes #n)
• to-tickets: defines how to split a spec into parts
• README: "Where specs and tickets live"

Repos set up on 4.5.0: rerun /hb:setup to refresh the seed.

Full change list in #20.