[P3] feat(doctor): Warn when an enabled plugin isn't installed

[github-actions]

Users who mistype a plugin name in doberman plugins enable get a success message, and nothing tells them later that no installed package provides that plugin.

Problem

plugin_config.enable (src/doberman/engine/plugin_config.py:130) checks only the name's format and then saves it. plugins_enable in src/doberman/cli/main.py prints "Enabled ..." either way. doberman doctor is where you'd expect to catch this, but run_checks has no plugin row (src/doberman/cli/doctor.py:584-601). This is tracked in #719.

Impact

Nothing fails open. An unmatched name never loads anything. But a user can think a plugin protects them when it does not. Today, the only way to spot this is to read doberman plugins list yourself. I couldn't measure how often it happens because the CLI sends no telemetry.

Solution

Add a non-critical _safe_check("Plugins", False, ...) to run_checks. It compares plugin_config.enabled_plugins() with the entry-point names from registry._iter_entry_points across ALL_GROUPS, the same name-only loop that plugins list uses, so no plugin is ever imported or loaded. It prints [warn] enabled but not installed: <name>, and it passes when every enabled name is installed or nothing is enabled. For tests in tests/unit/test_cli_doctor.py, the existing enable_plugins fixture points DOBERMAN_PLUGINS_FILE at tmp_path. Stretch goal: plugins enable writes the same warning to stderr.

Expected impact

I can't give a credible estimate because there's no usage data for this CLI. After the fix, every enabled but uninstalled name shows up on the next doberman doctor run instead of going unnoticed.

• Actionability: immediately_actionable
• Inbox status: ready
• Signals behind it: 1

Open the report in PostHog

Filed automatically from the PostHog Self-driving inbox. It mirrors the finding; the fix is a maintainer's call.