[copilot.just] Fix path encoding bugs and edge cases

[chicks-net]

Multiple review comments identified bugs in .just/copilot.just:

• sed -i not portable (13/15 PRs) - BSD vs GNU incompatibility at line 208. Fix: use temp-file approach for cross-platform compatibility
• copilot_rollback lossy path reconstruction (12/15 PRs) - Using _ for / replacement is ambiguous. Fix: use URL-encoding or base64
• copilot_pick ambiguous line-number lookup (11/15 PRs) - Matches by line number alone, not path + line. Fix: match by both
• Predictable temp file paths (4/15 PRs) - Use mktemp instead of hardcoded /tmp/copilot_count_*
• gum spin serialization fragile (8/15 PRs) - Using declare -f can break with complex function state

Identified in code reviews across 15 PRs using gh-process v5.8.