test: add directive discoverability regression tests

[buildingjoshbetter]

Summary

Add 17 regression tests covering all 4 Claude-facing directive discoverability surfaces, preventing silent degradation in future PRs.

Fixes #568

Root Cause

Zero tests existed to verify that CLAUDE_TEMPLATE.md, MCP instructions, tool schemas, and session start hook correctly expose directive functionality to Claude. Any future edit to these surfaces could silently break directive discoverability.

Fix

New test file tests/test_directives_discoverability.py with 17 tests across 4 surfaces:

Surface 1 — CLAUDE_TEMPLATE.md (4 tests):

• Template mentions directives
• Auto-Store section has directive=True guidance
• Template includes trigger phrases (always, never)
• Auto-Recall section mentions directives

Surface 2 — MCP instructions (5 tests):

• Directives section exists
• Storing section cross-references directives
• Trigger phrases (always do, never do, from now on)
• Management guidance (truememory_forget)
• Contradiction handling

Surface 3 — Tool schemas (5 tests):

• truememory_store has directive param with default False
• truememory_store docstring documents directive
• truememory_directives exists and has docstring
• truememory_forget exists
• truememory_forget has alwaysLoad meta

Surface 4 — Session start hook (3 tests):

• Directives block appears before context block
• Directive content not duplicated in context block
• Zero directives produces no directives block

Dependency Impact

• New test file only — no production code changes
• Tests import from truememory.mcp_server and truememory.ingest.hooks.session_start

Test Plan

• All 17 tests pass: pytest tests/test_directives_discoverability.py → 17 passed
• Full suite: 1061 passed, 6 skipped, 0 new failures