chore: port portable .claude/skills

[constk]

Problem

Teller has Claude skills directing how the agent reviews, plans, and writes code. The portable subset should ship with the template; Teller-specific skills (financial-review, eval-regression triage) should not.

Proposed solution

Audit .claude/skills/ in Teller. Port skills that are project-agnostic (e.g. PR review checklists tied to the harness, planning skills, security review). Drop skills that reference financial domain or Teller-specific files. Each ported skill gets reviewed for any leftover Teller references and edited.

Acceptance criteria

• Each ported skill mentions no Teller-specific concepts.
• Skills index (if any) updated.
• docs/HARNESS.md (docs: HARNESS, INVARIANTS, BOUNDARIES, DEVELOPMENT, EVAL_HARNESS, SECURITY, ARCHITECTURE skeletons #25) lists the included skills.

Priority rationale

Medium: skills add polish to the LLM coding experience; they're not load-bearing for code quality (CI is).

Depends on

#15

[constk]

Auto-close did not fire from the squash-merge body. Marking closed retrospectively — the work shipped weeks ago and the project board's Done column reflects the actual state once the project-item move that follows this comment lands.