Plugin manifest format, registry, and enabled-plugins config

[verkligheten]

Parent Epic

Part of #32 — Plugin system: toggleable internal subsystems

Blocks #35 (declarative surfaces) and #36 (hooks + allow-entries). Independent of all other sub-issues.

Task

Define the plugin manifest format and a registry that loads enabled manifests, plus the enabled-plugins list in user config. This sub-issue adds the mechanism only — it wires nothing into install or render yet, so dist cannot change.

Location

• agent_notes/plugins/ — NEW package root
• agent_notes/plugins/<name>/plugin.yaml — NEW manifest per plugin (none created here; format defined + validated by a fixture)
• agent_notes/registries/plugin_registry.py — NEW
• agent_notes/domain/plugin.py — NEW Plugin dataclass
• agent_notes/services/user_config.py — add enabled_plugins read/default helper
• tests/unit/registries/test_plugin_registry.py — NEW

Changes

Manifest format

name: cost-report            # unique; matches directory name
description: <one line>
default: on | off            # enabled when user config has no explicit choice
skills:   [<skill-name>, ...]     # names in data/skills/
agents:   [<agent-name>, ...]     # names in data/agents/
rules:    [<rule-name>, ...]      # names in data/rules/
includes: [<include-name>, ...]   # shared include names (data/agents/shared/<name>.md)
hooks:    [{event: Stop, command: "...", requires: stop_hook}]
allow:    ["Bash(agent-notes cost-report)", ...]

requires on a hook or allow-entry names a backend.supports(...) capability; the entry is installed only when the backend has it. This is how the manifest composes with the existing capability axis (epic finding 2) rather than fighting it.

Plugin dataclass + registry

• Plugin holds the parsed manifest fields.
• plugin_registry.load() globs plugins/*/plugin.yaml, parses each, validates required fields (name, description, default), and rejects a manifest whose name != directory name.
• registry.enabled(user_config) returns the plugins that are on: explicit user choice wins; otherwise default.

User config

• enabled_plugins key in ~/.config/agent-notes/config: a {name: bool} map, absent by default.
• Helper: enabled_plugin_names(config, registry) -> set[str].

Not in this sub-issue

No manifest files, no installer/render wiring, no CLI. Those are #35, #36, #34. Keeping this pure means the equivalence gate is trivially satisfied.

Verification

uv run pytest tests/unit/registries/test_plugin_registry.py -v
uv run pytest tests/ -v

Dist: byte-identical. This sub-issue adds unreferenced code; nothing in the build path calls it yet.

[verkligheten]

Plugin manifest + registry + enabled_plugins config — delivered, merged into develop via PR #42 (1f2a7f3).