Initial scaffold + canonical Activity model

Context
-------
New connector suite that syncs UK brokerage activity (Trading212,
InvestEngine, Schwab email-parsed, CSV drop-folder) into Wealthfolio.
Lives outside finance/ intentionally — finance/ is untouched per the
plan at ~/.claude/plans/let-s-work-on-linking-temporal-valiant.md.

This change
-----------
- Poetry project with httpx, typer, bs4, dev tools (pytest, mypy strict,
  ruff, yapf).
- Canonical Activity + Account models with the 6 UK tax wrappers
  (ISA/SIPP/GIA/LISA/JISA/WORKPLACE_PENSION) and the 12 Wealthfolio
  activity types from docs/activities/activity-types.md on the upstream.
- Validation invariants: BUY/SELL need qty+price, DIVIDEND/DEPOSIT/etc
  need amount — raises early so providers can't silently emit broken
  rows.
- to_wealthfolio_csv_row() shape matches Wealthfolio's CSV import;
  primary sink path per the plan.

Test plan
---------
## Automated
- poetry run pytest -q  →  7 passed in 0.03s
- poetry run mypy broker_sync tests  →  Success: no issues found in 4 source files
- poetry run ruff check .  →  All checks passed!
- poetry run yapf --diff --recursive broker_sync tests  →  no diff

## Manual Verification
Not applicable — pure data model, no runtime behaviour.

Closes: code-thw.1

.gitignore

@@ -0,0 +1,16 @@
+__pycache__/
+*.py[cod]
+*.egg-info/
+.pytest_cache/
+.mypy_cache/
+.ruff_cache/
+.coverage
+.venv/
+venv/
+dist/
+build/
+*.db
+*.db-journal
+.DS_Store
+.env
+.envrc