feat: auto-install support for the Junie agent

[mvanhorn]

What does this PR do?

Adds JetBrains' Junie to the agents that install / uninstall auto-detect and wire,
so Junie users get the codebase-memory-mcp MCP server registered the same way Cursor,
Antigravity, OpenCode and the others already are. Closes #651.

Junie is detected by the presence of ~/.junie/, and install writes the MCP entry to
~/.junie/mcp/mcp.json using the standard mcpServers JSON format (the same writer
Cursor and Antigravity use). The entry is upserted, so re-running install does not
duplicate it, and uninstall removes exactly that entry while leaving the rest of the
config intact. The change is additive and gated on detection, so non-Junie users are
unaffected. Junie is also listed in the Detected agents: line and the
install --plan receipt.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

New tests in tests/test_cli.c: cli_detect_agents_finds_junie_issue651 (detection) and
cli_junie_mcp_install_issue651 (install writes a well-formed entry, a second install
stays idempotent, uninstall removes it). The cli suite passes: 102 passed, 0 failed.

Fixes #651