[feat] add version to semble command in mcp and instructions

[stephantul]

Closes #186

This PR adds the current semble version to the MCP and instructions so that agents do not call a newer version of semble.

[greptile-apps]

Confidence Score: 4/5

Source and editable installations can generate an MCP command that runs different code or fails to resolve.

• Published release installations receive consistent pinned configuration.
• Local installations are converted into index lookups using only the hard-coded version.
• The generated MCP server can therefore differ from the installed code or fail to start.

src/semble/installer/agents.py

_{Reviews (1): Last reviewed commit: "add versioning to semble" | Re-trigger Greptile}

[Pringled]

LLLLGTM

[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% <100.00%> (ø)
src/semble/installer/config.py	100.00% <100.00%> (ø)
src/semble/installer/installer.py	100.00% <100.00%> (ø)

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