feat: add first-class pip install support

[Drswith]

Summary

Add first-class pip install support as a managed package manager for Python-distributed agent CLIs. This allows Quantex to install, update, and uninstall pip-based agents with the same lifecycle operations as npm, bun, brew, cargo, and winget managed agents.

Key changes:

• Add pip as a managed install type in ManagedInstallType
• Add pip package metadata support in AgentPackageMetadata
• Create pip package-manager provider with install/update/uninstall operations
• Add pip availability detection with fallback chain (pip → pip3 → python -m pip)
• Add pipInstall() helper function for agent definitions
• Add pip to capabilities and doctor diagnostic output
• Migrate Mistral Vibe from unmanaged binaryInstall('pip install mistral-vibe') to managed pipInstall('mistral-vibe')

Linked Artifacts

• Issue: DRS-26
• ADR: N/A
• OpenSpec: openspec/changes/add-pip-package-manager/
• Discussion: N/A

Validation

• bun run memory:check
• bun run lint
• bun run format:check
• bun run typecheck
• bun run test (if behavior changed)
• Not run, explained below

Release Intent

• Release: minor - user-facing feature

Docs Updated

• openspec/...
• Not needed
• docs/...
• Follow-up issue or OpenSpec change created instead

Scope Check

• I did not add a new ad hoc root-level Markdown file.
• I updated the relevant issue, ADR, spec, runbook, or captured the missing doc work as follow-up.
• I did not silently expand project scope without recording it explicitly.

Closure Check

• Working tree was clean after commit.
• Branch was pushed and this PR is the active delivery artifact.
• OpenSpec change is not needed, still active by design until merge, already archived, or queued for agent-driven archive closure.
• Release is not applicable, delegated to release automation, or verified.

Notes

• pip detection uses a fallback chain: pip → pip3 → python -m pip → python3 -m pip
• pip is a managed installer but not a self-upgrade provider for Quantex itself
• The OpenSpec change will need to be archived after this PR merges

Linear Issue: DRS-26