docs: document missing pipeline filter options (branch, build-reason, expression)

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Documentation Freshness Audit

This audit found the following inconsistency between code and documentation:

Findings

Area	Issue	File(s)
Pipeline trigger filters	branch, build-reason, and expression filter options are fully implemented in PipelineFilters but absent from user-facing examples	docs/front-matter.md, prompts/create-ado-agentic-workflow.md

Details

The on.pipeline.filters YAML examples in both user-facing docs only showed source-pipeline and time-window:

filters:
  source-pipeline: "Build*"
  time-window:
    start: "09:00"
    end: "17:00"

Three additional fields on PipelineFilters (src/compile/types.rs) are fully implemented and lowered by lower_pipeline_filters() in src/compile/filter_ir.rs, and are referenced in the developer-focused docs/filter-ir.md — but were absent from user-facing examples:

• branch — GlobMatch on $(Build.SourceBranch) (TriggeringBranch fact). Useful for filtering pipeline-completion triggers to specific branches of the upstream pipeline.
• build-reason — include/exclude by build reason (same semantics as the PR filter variant). Already validated at compile time for conflicting include/exclude.
• expression — raw ADO condition escape hatch (same as pr.filters.expression, already documented in the Expression Escape Hatch section of front-matter.md).

An AI agent authoring a pipeline using the prompts/create-ado-agentic-workflow.md prompt would have no way to discover these filter options.

Applied Fixes

• Added branch, build-reason, and expression to the on.pipeline.filters YAML example in docs/front-matter.md
• Added branch, build-reason, and expression to Step 11's pipeline trigger filters example in prompts/create-ado-agentic-workflow.md
• Clarified in the filter validation table that build-reason conflict detection applies to both PR filters and pipeline filters

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This pull request was created by the automated documentation freshness check.

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