#807 Docs: document .? optional select for null-safe CEL data access

What changed

swamp-club/swamp#783 adds .? (optional select) support to CEL expressions. When data.latest() or data.version() returns null for a missing instance, users can chain .? to get null instead of a thrown error:

# Returns null if data doesn't exist (instead of throwing):
findings: ${{ data.latest('factory', 'code-review').?attributes.?findings }}

# With inline default:
findings: ${{ data.latest('factory', 'code-review').?attributes.?findings.orValue([]) }}

Pages that need updating (swamp-club repo)

• content/manual/reference/cel-expressions.md — Add a section on .? optional select syntax with examples. Currently documents data.latest() as "returns null" but does not explain how to chain through null safely.
• content/manual/reference/data-querying.md — Add .? note in the CEL shortcuts section.
• content/manual/reference/data-outputs.md — Add .? variants to the data function table.

Key points to document

• .? is an explicit opt-in — regular . still throws on null (catches real bugs)
• .orValue(default) provides an inline default value
• Use .? when data might not exist (e.g., prior-cycle artifacts in rework loops)
• Use . when data must exist (dependent step outputs)