Schedule production-shaped frozen perf daily

[bill-ph]

Summary

• run posthog_frozen_perf for the daily scenario-dev schedule instead of full-suite
• preserve selectable manual runs with full-suite as the dispatch default
• define scenario selection once at job scope and document the new schedule
• cover the schedule, selector, and removed full-suite fallback in the workflow contract test

Testing

• just test-scenario
• go test -count=1 ./tests/mw-dev/scenario ./tests/mw-dev ./tests/perf/publishercli
• just lint
• git diff --check

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	0	1	0
E2E/journey files	0	0	0
Workflow files	0	1	0

Signals

• Test cases: +0 / -0
• Assertions: +0 / -0
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 0
• E2E/journey retry lines added: 0

Coverage risk: neutral or increased

No coverage-reduction warnings detected.