feat: add SOCRATICODE_PROJECT_ID for shared indexes across git worktrees

[cstuncsik]

Summary

When working with git worktrees (or any setup where the same codebase lives in multiple directories), each path currently generates a unique Qdrant collection. This means the same codebase gets indexed redundantly for every worktree.

This PR adds a SOCRATICODE_PROJECT_ID environment variable that overrides the path-based project ID. All directories sharing the same value use the same Qdrant collections (codebase, codegraph, context).

Use case

"socraticode": {
  "command": "npx",
  "args": ["-y", "socraticode"],
  "env": {
    "SOCRATICODE_PROJECT_ID": "my-project"
  }
}

With this config, agents running in /repo/main, /repo/worktree-feat-a, and /repo/worktree-fix-b all share one index — no redundant embedding or storage.

Changes

• src/config.ts: projectIdFromPath() checks SOCRATICODE_PROJECT_ID before hashing the path. Validates the value matches [a-zA-Z0-9_-]+.
• tests/unit/config.test.ts: 6 new tests covering the env var override, validation, whitespace trimming, and fallback behavior.
• README.md: Documents the new env var in the Indexing Behavior table.

Why this is safe

• Zero behavioral change when the env var is unset (default path)
• The explicit ID goes through the same collectionName() / graphCollectionName() / contextCollectionName() functions
• Validation ensures the value is always Qdrant-friendly
• File watcher and lock mechanisms work unchanged since they already use projectIdFromPath()