chore(deps): remove pyjwt constraint-dependencies when mcp bumps its floor

## Context

Issue #266 adds `constraint-dependencies = ["pyjwt>=2.13.0"]` to `agent/pyproject.toml` as a workaround for PYSEC-2026-175/177/178/179. This constraint is needed because `mcp` (v1.27.1) transitively pulls in pyjwt but hasn't bumped its own floor to >=2.13.0 yet.

## When to action

When `mcp` releases a version whose own dependency tree resolves pyjwt >=2.13.0 naturally (i.e., the constraint becomes redundant).

**How to check:** After bumping `mcp` in `agent/pyproject.toml`, temporarily remove the constraint and run `uv lock`. If pyjwt resolves to >=2.13.0 without the constraint, it's safe to remove.

## What to do

1. Remove the `pyjwt>=2.13.0` line from `[tool.uv] constraint-dependencies` in `agent/pyproject.toml`
2. If no other entries remain, remove the entire `[tool.uv]` section
3. Run `uv lock` to regenerate
4. Verify `osv-scanner` still passes (pyjwt should be >=2.13.0 from mcp's own deps)

## Acceptance Criteria

- [ ] `constraint-dependencies` for pyjwt removed from `agent/pyproject.toml`
- [ ] `agent/uv.lock` regenerated cleanly
- [ ] `osv-scanner` pre-push hook still passes