feat(store): add structured store-prune previews for automation

[TyceHerrman]

Depends on — complete store-prune serialization with every CAS/index writer.

Concrete consumer — tw93/Mole#1421

Problem

aube store prune --dry-run doesn't give automated cleanup enough information
to act safely. It omits paths the real prune may change and doesn't report the
total space it could free.

• aube store path prints only <storeDir>/v1, but prune also mutates the GVS
  and opening Store can migrate <cacheDir>/index. See
  store.rs,
  gvs_registry.rs, and
  aube-store/src/lib.rs.
• GVS dry-run locking can create .prune.lock, so preview also needs the full
  writer-exclusion protocol. See
  gvs_registry.rs.
• Human dry-run omits GVS bytes, a combined estimate, and structured paths.
  aube doctor --json reports paths but also runs unrelated health checks. See
  store.rs and
  doctor.rs.

Proposal

Add JSON output to the existing preview.

aube store prune --dry-run --json

The output should include

• every mutation root with an absolute path and kind
• planned actions such as legacy index migration
• GVS entries and bytes, CAS files and bytes, and stale project-record count
• combined reclaimable bytes
• structured warnings

Byte fields must identify upper bounds and avoid obvious hardlink double
counting.

Requirements

• Root resolution matches real prune for configured, environment, workspace,
  XDG, relative, ~, and relevant symlink paths.
• Every new mutation root also appears in JSON output.
• Preview doesn't delete, migrate the legacy index, initialize .projects, or
  make another persistent layout change.
• The complete prune/writer exclusion protocol protects one candidate snapshot.
• JSON mode emits one document on stdout without progress or human text.
• Failure doesn't delete anything, exits nonzero, and uses Aube's normal codes.

Example

Scheduled jobs could prune shared build runners or managed developer machines
only above a size threshold. disk cleaners like Mole could also validate every root before running
the serialized prune and skip cleanup when preview fails.

AI-assisted — Tool: Codex; model: unavailable/unavailable; version: unavailable.

[jdx]

Implemented in #1321.

This adds:

• a store-wide shared-writer/exclusive-prune lease held across GVS, index, and CAS planning/application
• one immutable plan used by human preview, JSON preview, and real prune
• machine-readable mutation roots and planned actions, including lexical and resolved paths
• GVS/CAS/stale-record counts plus hardlink-deduplicated byte upper bounds
• dry-run behavior that does not migrate the legacy index or initialize the project registry

One deliberate exception to strictly zero filesystem changes: stable maintenance lock files may be created so the preview can serialize with concurrent writers. Both lock paths are reported in mutationRoots for whitelist validation.

Validation passed: cargo fmt, clippy with warnings denied, the full Rust test suite, and all 18 store BATS tests.

AI-assisted — Tool: Codex; model: unavailable/unavailable; version: unavailable.