closes #545)

Eighth slice of the Tech Foundation Hardening epic (#538). Closes the
last UX gap on the v0.7.5 Wave 1 list.

The README's old 5-step quickstart had users (1) install (2) run the
interactive wizard (3) seed sample DuckDB data (4) hand-author a sync
YAML (5) run. Steps 2 and 4 were the friction — every newcomer typed
something different and roughly half mistyped. Replaces the path with
a single command:

  $ drt init --template duckdb_to_rest

Three curated templates ship in this PR (per #545's scoping):

- duckdb_to_rest      — DuckDB → REST API POST (httpbin.org, no accounts)
- postgres_to_slack   — PostgreSQL → Slack webhook (operational alerts)
- duckdb_to_hubspot   — DuckDB → HubSpot contacts upsert

Each template is a static, self-contained sync YAML living under
``drt/cli/templates/syncs/``. ``drt init --template list`` enumerates
them; ``drt init --template <name>`` writes ``syncs/<name>.yml`` plus
a minimal ``drt_project.yml`` + ``.drt/.gitignore`` shell and prints
per-template next-steps text (env vars to set, sample tables to seed).

Design notes
------------

- **Static files, not Jinja-rendered.** The on-disk template is what
  the user gets — no per-invocation substitution. Edits are then the
  user's domain. Keeps the scaffolding step boring and predictable.
- **Project shell is created if missing, never overwritten.** Running
  ``--template <name>`` in a directory with an existing ``drt_project.yml``
  appends only the new sync file; the project metadata stays put.
- **Registry lives at ``drt/cli/_init_templates.py``.** Each
  ``TemplateInfo`` carries name + description + next-steps tuple, so
  the CLI handler can render both ``--template list`` and the post-
  scaffold guidance from one source of truth.

README quickstart
-----------------

Rewritten end-to-end:

- New quickstart fits in one ``bash`` block: install + ``init --template``
  + seed + run. Three commands, copy-paste friendly.
- "Other starter templates" subsection lists the other two and points
  at ``--template list`` for the live set.
- "Customizing your sync" subsection demotes the interactive wizard
  to advanced usage — it still exists, just not the default path.
- Links added to ``docs/connectors/`` and to the new
  ``drt sources --detailed`` / ``drt destinations --detailed`` flags
  from #543, both as a discovery hint above the Connectors table and
  in the new "Customizing" subsection.

Tests
-----

``tests/unit/test_cli_init_templates.py`` — 9 tests:

- ``--template list`` exits 0 and surfaces every registered template
- Per-template (parameterized × 3): scaffolds the expected project
  shell, sync file lands under ``syncs/``, next-steps text prints
- ``--template`` does not overwrite an existing ``drt_project.yml``
- ``--template invalid_name`` exits 1 with a pointer to ``list``
- **Validation parity (parameterized × 3)**: each shipped template
  YAML parses as a ``SyncConfig`` via Pydantic. Fails fast if a
  template is shipped that ``drt validate`` would reject — i.e. the
  "ready to run" bar the issue calls for.

Local verification
------------------

  pytest tests/                  → 1185 passed, 1 skipped, 7 deselected
  ruff check drt tests           → All checks passed
  mypy drt                       → Success: no issues in 96 files

Out of scope (follow-ups)
-------------------------

- Quickstart GIF / asciinema (#282) lives in v0.8. This PR rewrites
  the text that the GIF will eventually record.
- README.ja.md updated in a follow-up commit so the i18n-sync marker
  hash can reference this commit.

Refs: #538 (epic), #545 (this issue), #543 (companion UX work —
``--detailed`` flags discovered from the new Connectors note), #282
(future GIF), #297 (v0.9 plugin system — third-party connectors can
register templates the same way).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>