perf(copilot): reduce tool schema token cost by 34% (#12398)

## Summary

Reduce CoPilot per-turn token overhead by systematically trimming tool
descriptions, parameter schemas, and system prompt content. All 35 MCP
tool schemas are passed on every SDK call — this PR reduces their size.

### Strategy

1. **Tool descriptions**: Trimmed verbose multi-sentence explanations to
concise single-sentence summaries while preserving meaning
2. **Parameter schemas**: Shortened parameter descriptions to essential
info, removed some `default` values (handled in code)
3. **System prompt**: Condensed `_SHARED_TOOL_NOTES` and storage
supplement template in `prompting.py`
4. **Cross-tool references**: Removed duplicate workflow hints (e.g.
"call find_block before run_block" appeared in BOTH tools — kept only in
the dependent tool). Critical cross-tool references retained (e.g.
`continue_run_block` in `run_block`, `fix_agent_graph` in
`validate_agent`, `get_doc_page` in `search_docs`, `web_fetch`
preference in `browser_navigate`)

### Token Impact

| Metric | Before | After | Reduction |
|--------|--------|-------|-----------|
| System Prompt | ~865 tokens | ~497 tokens | 43% |
| Tool Schemas | ~9,744 tokens | ~6,470 tokens | 34% |
| **Grand Total** | **~10,609 tokens** | **~6,967 tokens** | **34%** |

Saves **~3,642 tokens per conversation turn**.

### Key Decisions

- **Mostly description changes**: Tool logic, parameters, and types
unchanged. However, some schema-level `default` fields were removed
(e.g. `save` in `customize_agent`) — these are machine-readable
metadata, not just prose, and may affect LLM behavior.
- **Quality preserved**: All descriptions still convey what the tool
does and essential usage patterns
- **Cross-references trimmed carefully**: Kept prerequisite hints in the
dependent tool (run_block mentions find_block) but removed the reverse
(find_block no longer mentions run_block). Critical cross-tool guidance
retained where removal would degrade model behavior.
- **`run_time` description fixed**: Added missing supported values
(today, last 30 days, ISO datetime) per review feedback

### Future Optimization

The SDK passes all 35 tools on every call. The MCP protocol's
`list_tools()` handler supports dynamic tool registration — a follow-up
PR could implement lazy tool loading (register core tools + a discovery
meta-tool) to further reduce per-turn token cost.

### Changes

- Trimmed descriptions across 25 tool files
- Condensed `_SHARED_TOOL_NOTES` and `_build_storage_supplement` in
`prompting.py`
- Fixed `run_time` schema description in `agent_output.py`

### Checklist

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
  - [x] All 273 copilot tests pass locally
  - [x] All 35 tools load and produce valid schemas
  - [x] Before/after token dumps compared
  - [x] Formatting passes (`poetry run format`)
  - [x] CI green