tether 0.1.0a7

Fixes from the second review. States separate content from address (VOLATILE_KEYS / content_state): Iceberg metadata rewrites and Neon checkpoints no longer show as drift or mint duplicate pins; git dirty applies only to the checked-out ref. fork() onto an existing name resets it on every backend, Neon included (restore by head, sibling branch once pins hang off it). Stale detection is per object from recorded base states. Working-ref names carry a key digest and pin ids are 16 hex; earlier alphas' pins and branches are not recognised -- re-commit and new. apply_new verifies before moving the working copy and records successful forks before raising; git new keeps commits on a tether/ branch instead of a detached HEAD; gc --prune-workspaces discovers live jj workspaces and git worktrees. import locator changes drop the old working branch. export/publish/import and lakeFS/Dolt/DuckLake are labelled experimental.