Fix out-of-workspace platform file mapping in add command

[hyericlee]

Summary

Fixed extractPlatformSuffix returning the wrong path suffix for out-of-workspace files, causing incorrect platform selection and doubled path segments.

Root Cause

When adding an external file like /tmp/ext/.claude/skills/commits/SKILL.md, extractPlatformSuffix walks path suffixes shortest-to-longest:

1. SKILL.md → no match
2. commits/SKILL.md → no match
3. skills/commits/SKILL.md → matches **/skills/**/* (claude-plugin) AND skills/**/* (openclaw). Returned immediately — never reaches step 4.
4. .claude/skills/commits/SKILL.md → would match .claude/skills/**/* (claude) ← correct match, never reached

The short suffix skills/commits/SKILL.md caused:

• Wrong platform selected (claude-plugin instead of claude)
• mapPathUsingFlowPattern with empty fromBase returned skills/commits/SKILL.md as relPath
• Combined with subdir = "skills" → skills/skills/commits/SKILL.md (doubled)
• File placed at wrong location; test assertions failed

Fix

Modified extractPlatformSuffix to prefer suffixes whose first component is a known platform root directory (.claude, .cursor, .cline, etc.) via the existing getPlatformDirLookup(). Shorter matches without a platform dir prefix are saved as fallback.

For the example above:

• skills/commits/SKILL.md matches but skills is not a platform dir → saved as fallback
• .claude/skills/commits/SKILL.md matches and .claude IS a platform dir → returned immediately

Downstream, the correct claude platform is selected, fromBase = '.claude/skills' properly strips the prefix, producing relPath = 'commits/SKILL.md', and the final path is skills/commits/SKILL.md (correct).

Test Results

• ✅ tests/core/add/add-flow-based-mapping.test.ts — all 12 tests pass (2 previously failing)
• ✅ Full test suite — zero regressions (only pre-existing source-resolution.test.ts failure from separate path→base migration issue, fixed in Fix path→base migration gaps in dependency resolution and validation #49)