Task manager — types & pure logic (`WorkItem`, `Ticket`, key/status/person functions)

[abdotop]

Depends on: none

Add the core types and pure functions the rest of the read-only aggregator
builds on — no I/O, no STORE calls, no providers yet.

• WorkItem, Ticket, Comment, Person, PersonRef, ProjectScope,
  Provider types
• normalizeKey / extractKey
• resolvePerson
• grouping (WorkItem[] → Ticket[] by key) + deriveStatus

See TASK_MANAGER_INTEGRATION.md §1–§4 for the exact shapes and the
deriveStatus/extractKey logic to implement.

Acceptance criteria

• All types compile, exported from one module
• extractKey covers all three sources (jira, github, discord)
• deriveStatus covers the merged/open/fallback-to-jira cases
• Unit tests against fixture WorkItem[] arrays (no real STORE data
  needed for this issue)