feat: implement cova exporter add/remove/list commands

[MrPointer]

📝 Summary

Users had no way to manage which exporters are configured short of editing config.yaml by hand. This adds
cova exporter add/remove/list commands for managing the agents list, discovering available exporters
(built-in + external on PATH), and understanding what each exporter does via a new info protocol operation.

🔑 Key Decisions

Info Protocol Operation

Added a new info operation to the exporter protocol spec. External exporters can self-describe by responding
to {"operation": "info"} with their name and description. Exporters that don't implement it get a graceful
fallback (name from binary, empty description) rather than an error.

Structured List Results

exporter.List returns a ListResult struct (split by built-in vs external, with configured status) instead
of writing formatted text. Presentation is handled by the Cobra command layer — the library layer has no
io.Writer dependency. This keeps the library reusable for non-CLI consumers.

No-Args Discoverability

cova exporter add with no arguments lists available exporters (same as list). This routing lives in the
Cobra layer since it's a CLI affordance, not library logic.

No PATH Validation on Add

Names are stored as-is without checking if a matching exporter exists. Invalid names surface at cova apply
time. This is intentional — the exporter may not be installed yet when registering it.

Worker Agent Fix

Separated into its own commit: the Go worker agent definitions were hardcoding go test ./... and
go build ./..., overriding the preloaded skills that wrap the project's task runner. Fixed by replacing
with a directive to use skills.

🔄 Before & After

Area	Before	After
Agent management	Manual config.yaml editing	cova exporter add/remove commands
Exporter discovery	None	cova exporter list shows built-in + external with descriptions
Exporter protocol	apply and remove operations	+ info operation for self-description
PATH scanning	Exact-name lookup only (GetProgramPath)	+ prefix scanning (FindProgramsByPrefix)
Config mutations	Subscriptions only	+ AddAgents/RemoveAgents with locked read-modify-write
Worker agents	Hardcoded go test/build commands	Defer to preloaded skills for project tooling