Add external holiday data provider support via holidays.provider

[Copilot]

Holiday data is currently only readable from the config file. This adds an encapsulated API hook so an external tool can supply holiday data instead, enabling dynamic/third-party holiday sources.

Changes

src/data.cpp

• getHolidayEntriesFromProvider(provider, range) — new static helper; invokes the configured tool as <tool> YYYY-MM-DD YYYY-MM-DD, parses JSON Lines output into (Datetime, name) pairs; malformed lines are silently skipped
• getHolidays(const Rules&, const Range&) — new range-aware overload; delegates to the provider when configured, falls back to config-file data otherwise
• getAllExclusions() — updated to call the range-aware getHolidays so provider holidays feed into the gaps/exclusions engine
• createHolidayMap() — updated to use the provider when holidays.provider is set
• getHolidays(rules) (no-range) — hardened to skip the holidays.provider key and catch date-parse errors from unexpected config entries

src/timew.h

• Declares the new getHolidays(const Rules&, const Range&) overload

test/holidays.t (new)

• Integration tests covering: provider output in summary, provider overrides config holidays, config holidays work without a provider, date range passed as CLI args, malformed JSON lines skipped

Usage

# timewarrior.cfg
holidays.provider = /path/to/my-holiday-tool

The tool is invoked as:

/path/to/my-holiday-tool 2025-01-01 2025-12-31

Each line of output must be a JSON object:

{"date":"2025-01-01","name":"New Year's Day"}

When holidays.provider is set, config-file holiday entries (holidays.<locale>.<date>) are ignored entirely.

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