feat: optional_depends_on for sequencing without skip-gating

[joshua-temple]

Problem

depends_on is a hard gate today: if the dependency is skipped because its triggers didn't match, the dependent callback is also skipped. run_policy: always lets the dependent run even when the dep was skipped — but it also runs when the dep failed, which isn't always desired.

For pipelines where a deploy should wait for a build only when the build actually runs, neither current option fits.

Proposed

Add optional_depends_on: [<callback>]:

• Dep ran and succeeded → wait, proceed
• Dep ran and failed → skip the dependent (same as hard depends_on)
• Dep was skipped because its triggers didn't match → proceed without waiting

Impact

Enables the "build conditionally rebuilds, deploy always runs reading prior or fresh artifact from state" pattern without escape hatches like run_policy: always that also tolerate dep failure.