test(llm9p): add tests for per-session compact and usage files

- session_compact_test.go (llm): Tests for Session.EstimatedContextTokens,
  SessionManager.Compact (not found, too short, replaces messages, resets
  tokens), SessionManager.ContextLimit and EstimatedContextTokens.

- session_compact.go (llmfs): Fix Stat().Length for SessionCompactFile
  (was 0 from BaseFile default; now returns fixed read-msg length).

- session_compact_test.go (llmfs): Tests for SessionCompactFile (read,
  read EOF, write no-op short history, write compacts, stat), for
  SessionUsageFile (format, EOF, read-only write, stat, dynamic content),
  and SessionDir.Children/Lookup wiring for compact and usage.

52 tests total, all pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

feat(llm9p): per-session compact and usage files for context window management

Add automatic context window compaction support to the per-session 9P API:

- session.go: Add Session.EstimatedContextTokens() (4 chars/token heuristic
  over current messages — more accurate than cumulative totalTokens for
  threshold decisions). Add SessionManager.Compact(ctx, id) which summarises
  the conversation via AskWithRequest then replaces session.messages with a
  compact 2-message exchange. Add SessionManager.EstimatedContextTokens(id)
  and SessionManager.ContextLimit() (200K for all Claude models).

- session_compact.go: New /n/llm/N/compact file. Write any content to
  trigger Compact() for that session. Follows the SessionModelFile pattern.

- session_usage.go: New /n/llm/N/usage file. Read returns
  "estimated_tokens/200000\n". Follows the SessionModelFile pattern.

- session_dir.go: Wire compact and usage into Children() and Lookup().

Also land two pre-existing uncommitted fixes:
- cli_client.go: Accept result messages with empty Result field
- protocol.go: Increase MaxMessageSize 8192→65536 for large system prompts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>