fix: unify adoption marker format across CLI and plugin

[guodong-sq]

Summary

Replace the plugin's JSON wt-provisioned marker with the CLI's original directory-based wt/adopted approach. The marker is now a plain text file at <gitdir>/wt/adopted containing the context name.

CLI changes:

• Upgrade wt/adopted from empty sentinel to carry the context name (echo "$context" > wt/adopted)
• Add wt_read_adopted_context to read context name from marker
• No awareness of plugin's wt-provisioned format — the CLI only knows wt/adopted

Plugin changes:

• Rewrite ProvisionMarkerService to use wt/adopted plain text (Files.readString/writeString)
• Delete ProvisionMarker and ProvisionEntry data classes (Gson no longer needed for markers)
• Add fallback read from legacy wt-provisioned JSON during transition (plugin's own migration responsibility)
• Fix marker write timing — write only after successful metadata import + Bazel symlinks (matching CLI semantics)
• Simplify WorktreePanel tooltip (no multi-context provision display)
• Clean up legacy wt-provisioned on write

Design rationale:

Investigation confirmed provisioned_at and provisioned_by JSON fields were write-only (never read in production code), and multi-context provision tracking added complexity without value. The CLI's wt/ namespace directory with a simple text file is portable, extensible, and equally functional.

Backward compatibility:

• Old empty wt/adopted files still recognized (existence check unchanged)
• Plugin reads legacy wt-provisioned JSON via regex fallback during transition
• Legacy markers cleaned up by the plugin on next write

Test plan

• CLI unit tests pass: cd test && bats unit/wt-adopt.bats (22 tests)
• Plugin tests pass: cd wt-jetbrains-plugin && ./gradlew test
• Cross-tool: adopt via CLI → plugin shows as adopted
• Cross-tool: provision via plugin → wt list shows as adopted
• Old empty wt/adopted files handled gracefully

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