feat: Add dual-tag system for meta-observation control (#153)

* feat: Add dual-tag system for meta-observation control

Implements <private> and <claude-mem-context> tag stripping at hook layer
to give users fine-grained control over what gets persisted in observations
and enable future real-time context injection without recursive storage.

**Features:**
- stripMemoryTags() function in save-hook.ts
- Strips both <private> and <claude-mem-context> tags before sending to worker
- Always active (no configuration needed)
- Comprehensive test suite (19 tests, all passing)
- User documentation for <private> tag
- Technical architecture documentation

**Architecture:**
- Edge processing pattern (filter at hook, not worker)
- Defensive type handling with silentDebug
- Supports multiline, nested, and multiple tags
- Enables strategic orchestration for internal tools

**User-Facing:**
- <private> tag for manual privacy control (documented)
- Prevents sensitive data from persisting in observations

**Infrastructure:**
- <claude-mem-context> tag ready for real-time context feature
- Prevents recursive storage when context injection ships

**Files:**
- src/hooks/save-hook.ts: Core implementation
- tests/strip-memory-tags.test.ts: Test suite (19/19 passing)
- docs/public/usage/private-tags.mdx: User guide
- docs/public/docs.json: Navigation update
- docs/context/dual-tag-system-architecture.md: Technical docs
- plugin/scripts/save-hook.js: Built hook

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* fix: Strip private tags from user prompts and skip memory ops for fully private prompts

Fixes critical privacy bug where <private> tags were not being stripped from
user prompts before storage in user_prompts table, making private content
searchable via mem-search.

Changes:

1. new-hook.ts: Skip memory operations for fully private prompts
   - If cleaned prompt is empty after stripping tags, skip saveUserPrompt
   - Skip worker init to avoid wasting resources on empty prompts
   - Logs: "(fully private - skipped)"

2. save-hook.ts: Skip observations for fully private prompts
   - Check if user prompt was entirely private before creating observations
   - Respects user intent: fully private prompt = no observations at all
   - Prevents "thoughts pop up" issue where private prompts create public observations

3. SessionStore.ts: Add getUserPrompt() method
   - Retrieves prompt text by session_id and prompt_number
   - Used by save-hook to check if prompt was private

4. Tests: Added 4 new tests for fully private prompt detection (16 total, all passing)

5. Docs: Updated private-tags.mdx to reflect correct behavior
   - User prompts ARE now filtered before storage
   - Private content never reaches database or search indices

Privacy Protection:
- Fully private prompts: No user_prompt saved, no worker init, no observations
- Partially private prompts: Tags stripped, content sanitized before storage
- Zero leaks: Private content never indexed or searchable

Addresses reviewer feedback on PR #153 about user prompt filtering.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* feat: Enhance memory tag handling and indexing in user prompts

- Added a new index `idx_user_prompts_lookup` on `user_prompts` for improved query performance based on `claude_session_id` and `prompt_number`.
- Refactored memory tag stripping functionality into dedicated utility functions: `stripMemoryTagsFromJson` and `stripMemoryTagsFromPrompt` for better separation of concerns and reusability.
- Updated hooks (`new-hook.ts` and `save-hook.ts`) to utilize the new tag stripping functions, ensuring private content is not stored or searchable.
- Removed redundant inline tag stripping functions from hooks to streamline code.
- Added tests for the new tag stripping utilities to ensure functionality and prevent regressions.

---------

Co-authored-by: Claude <noreply@anthropic.com>

docs/public/docs.json

@@ -37,6 +37,7 @@
           "installation",
           "usage/getting-started",
           "usage/search-tools",
+          "usage/private-tags",
           "beta-features"
         ]
       },