[context-intelligence] Local events.jsonl mirror is unconditional — add a config option to disable it

Summary

hook-context-intelligence writes a near-duplicate of the session event stream to <session>/context-intelligence/events.jsonl unconditionally — the local disk write is always-on even when no CI server is configured. This is asserted by design:

• test_silent_by_default.py: "Disk always writes even without server config (always-on JSONL)"
• mount() docstring: "Always: ... LoggingHandler — writes events.jsonl + dispatches to CI server(s)"

The only way to avoid the duplicate today is to not compose the bundle at all.

Measured impact (one real machine)

• 1.5 GB of duplicate JSONL across 59 session files
• Each mirror is nearly identical in size to the parent events.jsonl (e.g. 75.77 MB vs 75.79 MB for one active session), doubling the cost of already-unbounded event logs (see #332 for the raw: true default that makes individual files huge, and #333 for the lack of any retention option)

Ask

A config knob to disable the local disk copy while keeping server dispatch, for users who compose the bundle only for the graph-server features, e.g.:

hooks:
  - module: hook-context-intelligence
    config:
      local_mirror: false

Related: #332, #333.

Start with test_silent_by_default.py and the mount() docstring, which describe the current always-on JSONL behavior. Trace how hook-context-intelligence reads its config and how LoggingHandler is composed. Add coverage for local_mirror: false while retaining server dispatch, and confirm the default behavior remains unchanged.