fix(desktop): find every bridge action source instead of listing them - #11990

`desktop_flow_contract` exists so "an added bridge action cannot skip its flow
validation route" — its own words. It failed at exactly that.

Three actions live in `DesktopAutomationBridge+Notifications.swift`. The
hand-maintained `ACTION_SOURCE_RELATIVE_PATHS` named only the base
`DesktopAutomationBridge.swift`, so the lint never read them and reported the
flows using them as referencing actions that do not exist:

    notifications-settings.yaml: unknown bridge action 'settings_notifications_snapshot'
    notifications-settings.yaml: unknown bridge action 'set_notification_settings'

Because `scripts/pre-push` lints every flow rather than the diff, that failed the
push gate on every branch cut from main, on a file the pusher never touched.

That instance is already fixed: `DesktopAutomationBridge+Notifications.swift` was
added to the list by hand on main while this was being written. This change is
about the recurrence, not the instance — the list is hand-maintained, so the next
bridge file added goes unlisted the same way and takes the push gate down with it
again.

Bridge sources are now discovered by their filename prefix. Scoped to that one
prefix deliberately: the pattern the lint matches, `name: "…"`, also appears in
`APIClient`, `AuthService` and `ChatBubble`, so scanning the tree would feed it
names that are not actions and weaken the check. The prefix belongs to the bridge
and nothing else, which is why the curated list is the right shape and only its
bridge entries needed to stop being hand-maintained.

Discovery finds three files where the list named two:

    Desktop/Sources/DesktopAutomationBridge.swift                (125 actions)
    Desktop/Sources/DesktopAutomationBridge+Notifications.swift  (3)
    Desktop/Sources/DesktopAutomationBridge+ChatFirst.swift      (0)

`+ChatFirst` registers nothing today. It is the case this is for: a file that
exists, is not listed, and would silently take actions out of validation the day
someone adds one to it.

Verification:
  python3 desktop/macos/scripts/desktop-flow-lint.py
    -> desktop-flow-lint OK (72 flows, 162 registered actions)
  python3 desktop/macos/tests/test_desktop_flow_contract.py      -> 3 passed
  python3 .github/scripts/test_run_checks.py                     -> 47 passed

  Tests proven by reverting to the hand-maintained list: two of the three fail
  and the lint reproduces the original two errors.

Wired into the check manifest in both lanes as `desktop-flow-contract-tests`.

Failure-Class: none

Desktop Swift Static & Test Contracts failed on
KernelTurnRecordedProjectionTests.testFaultHarnessResetUsesCredentialFreeControlClearOnceAndCompletesProjectionReset,
a kernel-projection fault-harness test in a subsystem this PR does not touch.
The same CI window produced two other unrelated Swift test failures on other
PRs, each a different test. No code change.