Add chat.message fallback for /dcp-compress when host skips command registration

[1kUnD0G]

Fixes #609

Problem

On recent OpenCode versions (post Effect-refactor), the slash-command list is built as a one-shot snapshot in packages/opencode/src/command/index.ts before plugin config hooks run. DCP registers /dcp-compress inside its config hook, so the registration lands after the snapshot was taken and never gets picked up: the command is missing from autocomplete and typing it manually just sends a plain user message.

Fix

Add a "chat.message" hook as a fallback path:

• When a user message's first non-ignored/non-synthetic text part starts with /dcp-compress, run the same manual-trigger flow as the slash command (handleManualTriggerCommand -> state.manualMode = "compress-pending" -> state.pendingManualTrigger), which the existing applyPendingManualTrigger consumes on the next messages transform.
• Normalize the stored text to the canonical /dcp-compress [focus] marker so downstream pruning/summary logic sees exactly what the slash-command path emits.
• Skip when state.pendingManualTrigger already exists (the real slash-command path ran and set it), when commands.enabled is false, or when compress permission resolves to deny.

This makes /dcp-compress work even when the host never registered the command, with zero behavior change on hosts where registration works normally.

Verification

• npm run build passes (tsup + tsc).
• Full test suite: 103/103 pass.
• Behavioral check against the built bundle:
  • plain messages are untouched;
  • /dcp-compress <focus> fetches session state, sets the pending trigger, and normalizes the marker;
  • a second invocation while a trigger is pending is skipped;
  • ignored/synthetic parts are not intercepted.
• README documents the snapshot-timing caveat and the opencode.json workaround for restoring autocomplete.