docs(site): fix pool default and add missing pipeline filter fields in front-matter reference (#644)

- Correct the pool example: replace stale 'AZS-1ES-L-MMS-ubuntu-22.04' string
  default with accurate vmImage: ubuntu-22.04 (Microsoft-hosted) form, and add
  commented alternatives for self-hosted string, self-hosted object, and 1ES.
- Add branch, build-reason, and expression to the pipeline.filters example; all
  three are present in PipelineFilters struct (types.rs) and the repo docs but
  were absent from the site page, while the 'Expression Escape Hatch' and
  'Filter Validation' prose sections already referenced them.

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

site/src/content/docs/reference/front-matter.mdx

@@ -18,8 +18,12 @@ engine: copilot # Engine identifier. Defaults to copilot. Currently only 'copilo
 #   model: claude-opus-4.7
 #   timeout-minutes: 30
 workspace: repo # Optional: "root", "repo" (alias: "self"), or a checked-out repository alias. If not specified, defaults to "root" when no additional repositories are listed in `repos:`, and to "repo" when one or more additional repos are checked out. See "Workspace Defaults" below.
-pool: AZS-1ES-L-MMS-ubuntu-22.04 # Agent pool name (string format). Defaults to AZS-1ES-L-MMS-ubuntu-22.04.
-# pool:                        # Alternative object format (required for 1ES if specifying os)
+pool:                          # Microsoft-hosted agent (default for standalone target)
+  vmImage: ubuntu-22.04        # defaults to ubuntu-22.04 when pool is omitted entirely
+# pool: MySelfHostedPool       # String form -- legacy shorthand for a self-hosted pool name
+# pool:                        # Self-hosted pool object form
+#   name: MySelfHostedPool
+# pool:                        # 1ES pool object form (set os: when needed)
 #   name: AZS-1ES-L-MMS-ubuntu-22.04
 #   os: linux                  # Operating system: "linux" or "windows". Defaults to "linux".
 repos:                           # compact repository declarations (replaces repositories: + checkout:)
@@ -84,10 +88,15 @@ on:                            # trigger configuration (unified under on: key)
       - main
       - release/*
     filters:                   # optional runtime filters (compiled to gate step)
-      source-pipeline: "Build*"
+      source-pipeline: "Build*" # glob match on upstream pipeline name (Build.TriggeredBy.DefinitionName)
+      branch: "refs/heads/main" # glob match on triggering branch (Build.SourceBranch)
       time-window:
         start: "09:00"
         end: "17:00"
+      build-reason:
+        include: [IndividualCI]  # only run when triggered by a commit push (not a schedule)
+        exclude: [Schedule]
+      expression: "eq(variables['Custom.Flag'], 'true')"  # raw ADO condition escape hatch
   pr:                          # PR trigger
     branches:
       include: [main]