fix(pst): suppress double browser-open for same URL (10s dedup)

[pstaylor-patrick]

Summary

• Fixes a bug where gh pr create followed immediately by gh pr edit --body-file fired the open-on-post hook twice for the same PR URL, opening the browser back-to-back.
• Root cause: both gh pr create and gh pr edit --body(-file) match the hook's posting-detection regex, so a create+immediate-description-update pattern produces two opens.
• Fix: 10-second recency dedup cache at ~/.cache/pst-hooks/open-on-post-dedup.json. Same URL within 10s is suppressed; genuine re-opens after the TTL pass through normally.

Implementation details:

• Atomic write via temp file + File.rename (POSIX-atomic, safe for concurrent hook processes)
• Self-healing: JSON::ParserError deletes the corrupt cache and starts fresh
• Cross-session scope is intentional and documented: two sessions suppressing each other within 10s is an acceptable trade-off (consequence = skipped tab, not a blocked open)
• TTL is 10s, not 60s: covers milliseconds-apart create+edit while minimizing the false-suppression window

Test plan

• Create a PR -- browser opens once, not twice
• Edit a PR description 15+ seconds after creation -- browser opens (not suppressed)
• Corrupt ~/.cache/pst-hooks/open-on-post-dedup.json manually -- next open self-heals (deletes file, opens normally)
• PST_NO_BROWSER=1 gh pr create -- no browser open

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