Modernize como: packaging, battery collection, SQLite storage and CI - #7

Four functional improvements on top of the modernized battery layer.

Multiple batteries:
- get_batteries() returns one entry per physical pack; get_battery() keeps
  returning a single aggregate for saving, summing capacities and taking the
  highest cycle count
- `como info` lists each pack separately when more than one is present

Power reporting:
- BatteryInfo gains power_mw and is_charging, computed consistently on every
  platform. Windows reports ChargeRate/DischargeRate in milliwatts and has no
  current reading at all, so deriving watts from voltage x current only worked
  on macOS and Linux
- macOS capacity now reads AppleRawMaxCapacity/AppleRawCurrentCapacity where
  present; on Apple Silicon the legacy keys report a percentage rather than
  mAh, which made health readings meaningless
- Windows raises when no battery is present instead of reporting zeroes, since
  each CIM query is wrapped in try/catch and yields an empty result

Time filtering:
- `como data --since 30d` (also w/m/y) limits output to a recent window

Exit codes:
- Failures raise ComoError, which the CLI turns into a non-zero exit instead
  of printing a message and returning success

The database was a zlib-compressed JSON blob handled through tablib, rewritten
in full on every save. Replace it with SQLite from the standard library.

- Schema records the full battery snapshot: time, capacity, cycles, voltage_mv,
  power_mw and is_charging. Previously only capacity and cycles were kept, so
  the readings shown by `como info` were never retained
- time is UNIQUE, making saves idempotent within a second
- --since is applied as a SQL WHERE clause rather than filtering in Python
- The file moves to como.db; databases from earlier versions are migrated on
  first use from both the old format and the old unsuffixed filename, keeping
  a .bak of the original
- tablib is no longer a runtime dependency; import and export use stdlib csv,
  and export/import now round-trip every column rather than silently dropping
  voltage, power and charge state
- Saving and importing report what actually changed instead of assuming the
  write succeeded, since duplicate rows are ignored by the UNIQUE constraint

76 tests covering the three modules, with no reliance on real battery hardware.

- test_battery.py: macOS plist parsing including the unsigned-amperage
  conversion and Apple Silicon raw-capacity keys, Linux sysfs across both the
  charge_* and energy_* driver variants, Windows CIM JSON parsing, multi-pack
  aggregation and platform dispatch
- test_core.py: sparkline rendering, SQLite schema and round-trips, migration
  from the legacy format and legacy path, --since parsing and filtering, and
  each command's success and failure paths
- test_cli.py: every subcommand through Click's CliRunner, including exit codes

Relative timestamps are used wherever a test depends on the current date, so
the --since cases cannot rot as they age.

- ci.yml runs ruff, ty and pytest as three parallel jobs on pull requests and
  on pushes to master. Limiting the push trigger to master keeps branch pushes
  from duplicating the pull-request run
- .pre-commit-config.yaml runs ruff lint and format, plus ty through uv so it
  resolves against the project environment
- pytest-cov is wired into the default pytest invocation with branch coverage,
  and CI uploads coverage.xml as an artifact
- dependabot.yml tracks uv and GitHub Actions dependencies weekly, with Python
  updates grouped into a single pull request. The alerts on master predate
  pyproject.toml and no longer describe what the project depends on
- mise.toml pins the toolchain for local development

Document the supported platforms, the SQLite database location and schema,
the migration from earlier versions, and the current command set. The previous
README still described the server upload workflow and the paxo-based install.