Add right-size-capability skill: tool vs MCP vs skill decision gate

[alexbaur]

What

Adds right-size-capability — a capability-level decision gate, sibling to add-supervisor's "is a supervisor warranted?" pre-gate. Before an agent-builder adds a capability, it decides whether it should be a UC-function tool, an MCP server, or an Agent Skill, and runs a falsification pass that argues against the option the user named before recommending. Builders routinely ask for the heavier mechanism ("write me an MCP server") when a governed UC function would do.

Design

• Invariant cascade (first gate that fires leads): knowledge-vs-action → governed-in-workspace-vs-served → tenancy/reuse, with a governance/observability tiebreak.
• Falsification pass runs before any recommendation: steelman the two options not chosen, test the leading option against its own necessary condition, and explicitly rebut the user's named artifact when the evidence lands elsewhere.
• Escalation trigger is structural (keyed to cascade outputs, not a self-reported "feels close"): inline self-critique by default with a decision-log seam; an independent single subagent steelman sits behind a flag (RIGHT_SIZE_ESCALATE=fork), off by default. It's a conditional single escalation — deliberately not a per-option debate swarm, so it stays on the right side of the same "don't over-orchestrate" gate add-supervisor applies.

Framing (native to this repo)

• Default = UC-function tool (governed by UC, in-workspace, traced as a tool call for free) → routes to uc-functions-ops.
• MCP server and Agent Skill are decision + guidance paths — the template doesn't scaffold them today, so the skill decides and points the builder at the mechanism rather than generating it. (A scaffolder for those would be a follow-up.)
• Self-contained and grounded in the Big Book operating principles (start narrow / expand deliberately; govern tools, data, actions centrally; design for observability) — no external references.

Changes

• plugin/skills/right-size-capability/SKILL.md — new skill
• plugin/skills/install_skills.sh — registered in SKILL_NAMES + both --list/help blocks

Testing

• Frontmatter valid; matches repo convention (name + description).
• install_skills.sh: bash -n clean; --list renders the new entry.
• Full suite: pytest tests -vv → 91 passed (Databricks CLI v1.16.0) — same as the post-Add supervisor pattern: /add-supervisor skill (custom LangGraph) #33 baseline; no test enumerates skills, so the install-list addition is not test-gated and nothing regresses.

Follow-ups (not in this PR)

• Wire it as a gate: reference it from uc-functions-ops ("When to use") and the scaffold's Phase 3 (Tools) — mirroring how add-supervisor is referenced from agentops-lifecycle Step 3.5. Held out so this PR stays a focused, reviewable addition.
• Merge-order note: an in-flight agentops-bigbook branch also appends to install_skills.sh SKILL_NAMES; whichever merges second will need a one-line conflict resolution there.

This pull request and its description were written by Isaac.