Redesign CausalPy method selection skill

[drbenvincent]

Summary

• Replace flat keyword routing with ordered intake, a text decision tree, and four output contracts (Matched / Ambiguous / Not identifiable / Not in CausalPy).
• Add disambiguation cards, a capability matrix, and a not-in-CausalPy reference so agents stop force-fitting unsupported designs.
• Add CITS routing via InterruptedTimeSeries with comparison series as predictors.
• Rebased onto current main and polished for review: trimmed YAML description into reference/triggers.md, documented staggered DiD untreated-period identification support (Warn when staggered DiD ATTs lack untreated-period support #949), and cross-linked causal-detective after a matched route.

Why now

choosing-causalpy-methods on main still uses flat Fast Routing. This PR fills the agent force-fitting gap and complements #952's causal-detective skill (post-selection validation).

Reviewer guide

1. Start with causalpy/skills/choosing-causalpy-methods/SKILL.md.
2. Read reference/decision_tree.md for the canonical routing algorithm.
3. Spot-check one disambiguation card (e.g. its_vs_sc_vs_did.md) and method_capability_matrix.md.
4. Confirm the capability matrix matches running-causalpy-experiments/SKILL.md output exceptions.

Out of scope

• Decision AI Hub publish (follow-up after merge).
• Deeper causal-detective integration beyond the Matched-route cross-link.

Test plan

• prek run on touched skill files
• Rebased onto current main
• CI on rebased branch
• Manual read-through: capability matrix rows match execution-skill exceptions

[read-the-docs-community]

Documentation build overview

📚 causalpy | 🛠️ Build #32840606 | 📁 Comparing eafc26a against latest (c107d4e)

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1 file changed

± 404.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (2f2685c) to head (1407a3b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #931   +/-   ##
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  Coverage   95.03%   95.03%           
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  Files          89       89           
  Lines       14105    14105           
  Branches      854      854           
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  Hits        13404    13404           
  Misses        489      489           
  Partials      212      212           

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