fix(client-runtime): Normalize runtime agent labels

[bryantderosier]

What does this implement or fix?

Adds one shared formatter for provider-native agent paths and prefixed labels so clients can present consistent, human-readable runtime agent names. For example, /root/security_review becomes Security review, and redundant Subagent: prefixes are removed.

This is PR 10 of 12 in GitHub stack pingdotgg#89. It targets feature/agent-persona-settings and depends on #85.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)
• This change requires a documentation update

How has this been tested?

The focused remaining-stack suite passed 234 tests across 10 files, including the shared subagent runtime presentation suite.

• Focused tests pass locally
• Existing behavior outside this layer remains covered
• The branch diff passes git diff --check

Screenshots or GIFs (Optional)

Not applicable to this shared client-runtime formatting layer.

Merge order

This PR is part of GitHub stack pingdotgg#89. Merge from bottom to top:

#75 -> #76 -> #78 -> #79 -> #80 -> #81 -> #83 -> #82 -> #85 -> #84 -> #86 -> #118

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[bryantderosier]

Closed: the two subagent label helpers here existed only to feed the Agents panel restyle in #86, which is dropped from this stack to keep upstream rebases clean. Neither is part of the rebuilt stack. If the restyle is wanted, both belong in a PR against pingdotgg/t3code.