fix(.trae): harden install/uninstall scripts after merged Trae support

[affaan-m]

Merged PR #985 introduced useful Trae support, but the install/uninstall scripts still have concrete safety and correctness problems that need a follow-up fix.

Tracked issues:

• .trae/uninstall.sh trusts manifest paths too broadly; entries should be bounded to the install root before rm / rmdir operations.
• .trae/install.sh rewrites .ecc-manifest on reinstall, which can orphan previously installed files and make uninstall incomplete.
• recursive skill installation and manifest recording are out of sync; nested files can be copied without corresponding manifest coverage.
• rules installation only copies rules/common/*.md, while the docs imply a broader rules tree is installed.

Expected fix:

• manifest entries must be normalized and rejected if they escape the target root
• reinstall must preserve or merge prior manifest coverage instead of truncating it
• copied file inventory and manifest inventory must match
• docs must reflect the actual rules copied, or the installer must copy the documented scope

Source context: merged PR #985 (Add Trae IDE support (.trae/)).

[affaan-m]

Opened #1013 to fix the real ownership bug in the Trae installer: preexisting target files were being added to .ecc-manifest and could later be deleted on uninstall. The PR also adds shell-level regression tests for nested skill/rule manifest coverage, reinstall idempotence, and symlink-escape rejection.