feat: Add reasonix and pi support

[Pringled]

This PR adds an install path for both reasonix and pi, which resolves both #79 and #170.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/semble/installer/agents.py	100.00% <ø> (ø)
src/semble/version.py	100.00% <100.00%> (ø)

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[greptile-apps]

Confidence Score: 4/5

Safe to merge after verifying whether Pi requires an explicit tools declaration in sub-agent frontmatter.

The Reasonix and Pi installer entries are well-structured and consistent with how other agents are integrated. The one concrete gap is pi.md lacking a tools field — every other agent file in this directory declares allowed tools, and without it the installed Pi sub-agent may silently be unable to run shell commands. If Pi's sub-agent system does not require an explicit tools list, the change is clean; if it does, users who install the Pi sub-agent will get a broken agent.

src/semble/agents/pi.md — the missing tools declaration is worth confirming against Pi's sub-agent spec before merging.

_{Reviews (1): Last reviewed commit: "Update test and docs" | Re-trigger Greptile}