fix(state): delete the session state file when a session is deleted

[Mukller]

Closes #557

Problem

DCP persists prune state per session at
$XDG_DATA_HOME/opencode/storage/plugin/dcp/{sessionId}.json, and nothing ever removed those files. Deleting a session left the file on disk forever, and loadAllSessionStats kept counting it — it aggregates every *.json in that directory, so the all-time totals drifted upward permanently.

The host already emits session.deleted carrying the session id, but createEventHandler only ever looked at message.part.updated:

if (input.event.type !== "message.part.updated") {
    return
}

so the delete case was unreachable.

Change

• deleteSessionState(sessionId, logger) in lib/state/persistence.ts unlinks the file and swallows the error, since the file may never have been written or may already be gone.
• createEventHandler handles session.deleted before the timestamp extraction, so it cannot perturb the compression-timing branch. If the deleted session is the one the plugin holds state for, state.sessionId is detached so the next request re-initialises instead of writing the file straight back.

Verification

tests/session-cleanup.test.ts, six cases:

• the state file is gone after session.deleted
• deleting one session leaves the active session's file and in-memory id alone
• deleting the active session detaches state.sessionId
• malformed delete events (properties missing, empty id) are harmless
• the message.part.updated timing branch still records compression starts
• unrelated events do not touch the storage directory

Three of the six fail on the current implementation with the orphaned state file survived.

Full suite 125 passing, tsc --noEmit clean, prettier --check clean.