perf: add regex cache, sync.Once templates, and atomic fail-fast

[indaco]

Description

Performance improvements across changelog generation and workspace execution:

• (changelog parser) Add thread-safe regex cache with sync.RWMutex to avoid recompilation of exclude/group patterns
• (changelog generator) Replace nil-check template caching with sync.Once for thread-safe contributor template initialization
• (workspace executor) Replace mutex-based fail-fast check with atomic.Bool and CompareAndSwap for lock-free signaling

Related Issue

• None

Notes for Reviewers

• None