feat: allow retaining cmperf CM2 temp files for debug

[cgrinds]

Add HARVEST_CMPERF_RETAIN_FILES so CM2 pb files can be kept for troubleshooting, with newest-N pruning.
Isolate temp dirs as
cmperf-{poller}-{uid}/{object} so different Unix users do not collide.

[Copilot]

Pull request overview

Adds an opt-in debugging mechanism for CMPerf CM2 protobuf downloads by retaining the newest N temp .pb files (via HARVEST_CMPERF_RETAIN_FILES) and reduces temp-dir collisions by isolating temp directories per poller and Unix user.

Changes:

• Introduces HARVEST_CMPERF_RETAIN_FILES parsing and temp-directory pruning logic (newest-N retention).
• Updates CMPerf temp directory layout to cmperf-{poller}-{uid}/{object} under the chosen temp root.
• Adds unit tests for retain parsing and pruning behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
cmd/collectors/cmperf/cmperf.go	Changes CM2 temp-dir path to include poller + UID + object for better isolation.
cmd/collectors/cmperf/cm2.go	Adds retention env var handling and pruning logic; integrates pruning into download/parse lifecycle.
cmd/collectors/cmperf/cm2_test.go	Adds tests covering retain parsing and pruning “keep newest N” semantics.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.