tools/mpy-triage: Add database layer and GitHub API wrapper

[andrewleech]

Summary

Adds the foundational database and GitHub API modules for mpy-triage, a CLI tool for detecting duplicate/related MicroPython issues and PRs using semantic search.

• db.py: SQLite connection management with WAL mode, schema initialization, sync state key-value store, and sqlite-vec extension loading.
• gh.py: gh CLI wrapper with paginated response parsing (handles concatenated JSON arrays), rate-limit detection with sleep-until-reset backoff, search queries with date range filtering, and PR diff fetching.
• Tests for both modules (27 tests total) using tmp_path fixtures and mocked subprocess calls.

Testing

uv run pytest tests/test_db.py tests/test_gh.py -v - 27 tests pass.
uv run ruff check src/ tests/ - clean.
uv run mpy-triage --help - works.

Generative AI

I used generative AI tools when creating this PR, but a human has checked the
code and is responsible for the code and the description above.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.46%. Comparing base (5c00edc) to head (5284bef).
⚠️ Report is 6 commits behind head on review/mpy-github-triage.
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Additional details and impacted files

@@                    Coverage Diff                    @@
##           review/mpy-github-triage      #32   +/-   ##
=========================================================
  Coverage                     98.46%   98.46%           
=========================================================
  Files                           176      176           
  Lines                         22784    22784           
=========================================================
  Hits                          22435    22435           
  Misses                          349      349           

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[github-actions]

Code size report:

Reference:  esp32/boards/SEEED_XIAO_ESP32C6: Add new XIAO board definition. [2dc2e30]
Comparison: tools/mpy-triage: Add gitignore and remove egg-info from tracking. [merge of 5284bef]
  mpy-cross:    +0 +0.000% 
   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
      esp32:    +0 +0.000% ESP32_GENERIC
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32