Fix nullable worktree inventory upserts

[chubes4]

Summary

• omit null normalized inventory values before database replace operations
• advance the inventory schema to version 2 so active installs rerun the additive schema definition and receive lifecycle-intent columns
• cover SQLite nullable upserts plus lifecycle-intent persistence after a version-1 table migration

Fixes #1014

Verification

• php tests/sqlite-worktree-inventory-contention.php
• php tests/worktree-add-lifecycle.php
• homeboy review lint --changed-only --placement=local --summary

AI assistance

OpenAI gpt-5.6-sol via OpenCode implemented and verified the nullable inventory upsert and additive schema migration. Chris remains responsible for the change.