feat: symlink preserved files instead of copying them

[raphi011]

Replaces the copy-based preserve system with symlinks so that edits to preserved files (e.g. .env) propagate instantly across all worktrees. The source of truth is now the repo root directory (containing .git/), and patterns are replaced with literal relative paths.

Breaking Changes

• preserve.patterns renamed to preserve.paths (literal relative paths, no globs)
• preserve.exclude removed entirely
• Existing worktrees with copied files are unaffected (symlinks only for new worktrees)

Test Coverage

• Unit tests added: 9 (TestLinkFile: 5, TestPreserveFiles: 4)
• Integration tests added: 4 (TestPreserveFiles: symlink creation, missing source, existing dest, edit propagation)
• Unit tests updated: TestValidatePreservePaths, TestMergeLocal_PreserveAppendDedup, config load tests
• Integration tests updated: TestCheckout_PreserveFiles, TestCheckout_PreserveNoOverwrite, TestCheckout_NoPreserveFlag
• Integration tests removed: TestCheckout_PreserveExclude (exclude config removed)

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/preserve/preserve.go	72.41%	5 Missing and 3 partials ⚠️
cmd/wt/config_cmd.go	50.00%	1 Missing and 1 partial ⚠️
cmd/wt/checkout_cmd.go	85.71%	0 Missing and 1 partial ⚠️
internal/config/config.go	0.00%	1 Missing ⚠️

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