Implement `dotagent plugin invoke` (currently a stub)

Plugins in dotagent are subprocesses that speak JSON over stdio (info, validate, invoke — see docs/reference/plugin-protocol.md). The daemon invokes them automatically, but when something goes wrong (silent failure, wrong output shape, unexpected exit code), the only debugging paths are:

• Run the plugin binary by hand and pipe JSON into stdin (works but tedious to construct payloads).
• Read the daemon logs after the fact, often without enough context.

dotagent plugin invoke <name> <payload> is declared in the CLI as the interactive debug path, but the implementation is a stub returning Err("plugin invoke <name> — not yet implemented"). Plugin development is meaningfully harder than it needs to be because of this.

Proposal

Make dotagent plugin invoke mirror what the daemon does, but interactively:

• Resolve the plugin binary the same way PluginClient does (PATH + plugin dir search).
• Accept <payload> as either a JSON literal or - (read from stdin).
• Pretty-print the plugin's response (status, output, suggested action).
• Honor the same env contract the daemon passes (AGENT_NAME, AGENT_HOME, etc.) — likely via --agent <name> / --schedule <id> flags that pull from a real manifest if available, or fall back to placeholders.

Acceptance criteria

• dotagent plugin invoke sink-file '{"event":"success","stdout":"hi"}' runs the plugin and prints its response.
• echo '{...}' | dotagent plugin invoke sink-file - works.
• When the plugin is not found, the error message points at the search path that was tried.
• Output uses the same human/JSON renderer convention defined in CLAUDE.md (no raw Debug dumps).
• docs/concepts/plugins.md gains a "Debugging a plugin" section.

Where to start

• crates/dotagent/src/commands/mod.rs::plugin_invoke — current stub.
• crates/dotagent-plugin/src/lib.rs — PluginClient, the existing invocation flow.
• docs/reference/plugin-protocol.md — JSON shape of each verb.

Non-goals

• Replacing the daemon's plugin lifecycle. This is a one-shot debug command.
• A REPL or interactive prompt. Single invocation, single response.