feat(workflow): H2→preview server auto-launch + /pf:preview slash command (Phase 2) #190

Job	Run time
Template build smoke (B1+B2 regression guard)	22s
Test hooks (unit)	12s
Agent inventory (144 target)	4s
Shell script lint	16s
Verify plugin structure (ubuntu-latest)	17s
Verify plugin structure (macos-14)	21s
	1m 32s

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