ci: disable scheduled workflow runs

[vyncint]

What & why

Nothing in this repository runs on a cron any more. The schedule: block is
commented out in every workflow that had one:

• install.yml (47 7 * * 1)
• pins.yml (17 6 * * 1)
• stress.yml (29 6 * * 1)

Every other trigger is untouched — push, pull_request, release,
workflow_dispatch and repository_dispatch all behave exactly as before.
The cron expressions are kept as comments rather than deleted, so restoring a
schedule is uncommenting one block, and the reasoning behind each chosen time
stays next to it.

Comments and prose that stated a cadence as current behaviour are corrected
in the same commit. Prose explaining why a cadence was chosen is left alone —
it still explains the block sitting right there, commented out.

pins.yml is now dispatch-only, so an upstream cuda-oxide or reconverge move
is noticed when someone runs it. CONTRIBUTING §9 said the workflow was
scheduled; it now says how to reach it. (§9 is a per-repo section, not part of
the generated §4–8.)

stress.yml's hunt job keeps || github.event_name == 'schedule' in its if:.
That branch is unreachable while the cron is commented out, and leaving it means
restoring the schedule stays a one-block uncomment.

What this does not do

• No CHANGELOG.md entry: nothing here is user-facing.
• No tests: this is workflow configuration, and its only real check is that
  the YAML still parses and every workflow keeps at least one trigger. Both
  were verified before pushing.
• Dependabot's weekly schedule is left alone — it is dependency management,
  not CI.

Checklist

• No issue linked — this is a direct maintainer request, not a tracked bug
• Tests added/updated — n/a, workflow configuration only
• No Rust changed, so fmt/clippy are unaffected
• All commits are signed off
• No AI attribution trailers
• CHANGELOG.md — n/a, not user-facing
• No snapshots touched