fix: restore object-tree and object-store READMEs

[pyramation]

Summary

Restores the original rich READMEs for @pgpm/object-tree and @pgpm/object-store that were accidentally overwritten in PR #70 with stripped-down versions from constructive-db.

The pgpm-modules READMEs are the canonical detailed versions with philosophical context, tables, detailed examples, and full API documentation. The constructive-db copies were simplified summaries that should not have been upstreamed.

Review & Testing Checklist for Human

• Verify the restored READMEs look correct (should match what was on main before PR feat: upstream pgpm-modules from constructive-db #70)

Notes

• No code changes, only documentation restoration

Link to Devin session: https://app.devin.ai/sessions/2cd4b3b9432a45acab518af28204ced2
Requested by: @pyramation

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