Copilot workspace.yaml: consolidate duplicate regex parsers and replace verbatim prompt-text matching; add tests

[shmuelie]

Summary

The Copilot session tooling contains two independently-written, regex-based
YAML parsers for the same workspace.yaml format, plus a fragile
string-matching heuristic — all without tests.

Details

• Get-CopilotLaunchPlan.ps1 (~lines 700-720) parses cwd/summary/name/
  branch out of workspace.yaml with one set of regexes.
• SessionMaintenance.ps1 Merge-CopilotSession (~lines 237-259) rewrites
  the name: field with a different, independently-written block-scalar
  handler.

Because neither uses a real YAML parser, an edge case (folded > scalars,
different indentation, quoted flow scalars, CRLF vs LF) may be read correctly in
one place and mis-parsed/corrupted in the other, and a fix in one won't apply to
the other.

• Get-CopilotLaunchPlan.ps1 ($ignoredSessionNames, ~line 700) excludes
  auto-generated maintenance sessions from auto-resume by matching the session
  name against verbatim hardcoded copies of the Copilot CLI's internal
  maintenance-prompt text. Any upstream wording/punctuation/localization change
  silently breaks the exclusion (maintenance sessions start appearing as
  resumable), with nothing to catch the drift.

Recommendation

• Extract one shared internal helper
  (Get-CopilotWorkspaceField / Set-CopilotWorkspaceField) used by both call
  sites.
• Prefer a stable signal (a session kind/purpose field, tag, or naming prefix)
  over verbatim prompt-text matching; if none exists, add a regression test that
  fails loudly when the upstream session shape no longer matches.
• Add unit tests covering quoted values, block scalars (|, |-, >, >-),
  and CRLF content.

Filed from an automated peer review (Skeptic persona); please validate before acting.