fix: store plugin traces in payload workspace

[Siddhant-K-code]

Summary

• resolve the hook trace store from a validated absolute cwd in the normalized hook payload when AGENT_TRACE_DIR is unset
• preserve explicit AGENT_TRACE_DIR precedence and legacy fallback behavior for missing or invalid payload directories
• add regressions for camelCase and snake_case payloads, process/payload cwd separation, env precedence, and invalid cwd values
• document the namespaced agent-strace:trace-analyst agent and bump plugin/package metadata to 0.94.1

Root cause

Marketplace hooks execute with the installed plugin directory as their process cwd. The hook store default was the relative path .agent-traces, so plugin-triggered sessions were written beneath the installation directory even though Copilot supplies the repository cwd in every official hook payload.

Validation

• PYTHONPATH=src python3 -m unittest discover -s tests -p 'test_*hooks.py' -v (62 passed)
• focused Copilot hook and plugin tests (12 passed)
• clean isolated local marketplace install reports agent-strace@agent-trace (v0.94.1); invoking the installed hook from the plugin directory creates the trace only at <payload cwd>/.agent-traces
• full suite: 1,906 tests run; 1,884 passed, 1 skipped, with 21 unrelated pre-existing failures/errors caused by macOS /var symlink path-safety checks in assignment/compliance tests