docs: teach the tour and the runbooks about flask

[ttncode]

What this changes

flask reached README.md and docs/PROVENANCE.md when it landed, but not
docs/tour/ or docs/runbook/. Those were not stale in the obvious way — the
tour says outright to run scaffold list rather than trust a list written into
it, and nothing anywhere claims a count of adapters. They were narrow: four
sentences describe a rule that flask is the first adapter to answer
differently, and a reader following them would look in the wrong file.

• docs/tour/08-adapters.md said an adapter runs "a framework's own
  generator". Flask ships none, so the package manager's project init stands in
  (uv init --bare, which writes a pyproject.toml and nothing else) and the
  overlay supplies the application. That is a fact about the mechanism, not
  about one adapter: it is what shows the overlay does not depend on a
  framework generator existing.
• docs/tour/01-toolchain.md is the page about where a version is pinned,
  and had two answers. Flask is a third: mise.toml pins uv and no python,
  because uv resolves its own managed interpreter and would install a
  mise-pinned one only to ignore it.
• docs/runbook/bump-a-toolchain-version.md listed where to look for a
  version. Searching the [tools] blocks for flask's python finds nothing, so
  the bullet now names .python-version and the --python argument in
  adapters/flask/adapter.env that sets requires-python at generation.
• docs/runbook/add-an-adapter.md told an author to reach for
  ADAPTER_POST_GENERATE "only once you've hit a real generator bug". That
  stays true for a framework generator, but where the generator writes only a
  manifest the dependencies are ordinary setup, and adapters/flask uses the
  field for exactly that. Its mise.toml section and its adapter.env sample
  gain the same one-clause correction.

No prose was rewritten, no section added, and no example replaced — each edit
extends a sentence that had become narrower than the code.

How it was verified

• bats tests/documentation.bats — 4/4
• mise run lint
• Every claim checked against the file it describes rather than from memory:
  adapters/flask/mise.toml pins uv and no python, .python-version reads
  3.13, and adapters/flask/adapter.env's generator carries --python 3.13.
• No line over 80 columns, matching the surrounding documents.

Checklist

• mise run lint passes
• mise run test-runner passes
• New behaviour has a test that fails without the change — n/a: no
  behaviour changes, this is documentation only
• Docs that describe changed behaviour were updated in the same commit
• No unrelated changes