feat(workflow): add json() template filter for webhook payload extraction

[genesis1tech]

Problem

The webhook bridge flattens only top-level payload keys — nested objects arrive as one stringified blob (crates/buzz-relay/src/bridge.rs). So a GitHub issues webhook gives a template exactly one handle, {{trigger.issue}}, and the only filters available were truncate() and npub. A workflow that wants the issue title and body has to dump the entire JSON object into the message.

Change

Adds a json(PATH) filter to apply_filter:

text: |
  New issue #{{trigger.issue | json(number)}}: {{trigger.issue | json(title)}}
  by {{trigger.issue | json(user.login)}}

  {{trigger.issue | json(body)}}

• Parses the value as JSON, then walks a dot-separated path. Numeric segments index arrays (json(labels.0.name)).
• String results render unquoted; other scalars render as JSON (41, true).
• Missing fields and null resolve to an empty string — payload shapes vary across providers and event types, so an absent optional field shouldn't fail the run.
• A value that isn't JSON at all is a TemplateError: that means the template targets the wrong field, and failing loudly is right.

No new dependencies (serde_json is already in the crate).

Test plan

Six unit tests in executor.rs covering string extraction, non-string scalars, nested paths, array indexing, missing/null → empty, non-JSON value error, and empty path error.

cargo test -p buzz-workflow     # 155 passed, 1 ignored
cargo clippy -p buzz-workflow --all-targets   # clean
cargo fmt -p buzz-workflow --check             # clean