refactor(cli): rename binary digstore -> dig-store with transitional dual-publish

[MichaelTaylor3d]

Task

Step [2] of the rename epic #703 (RELEASE-FIRST). Rename the primary CLI binary digstore -> dig-store to match the renamed repo, keeping the digs alias and ALL crate/package names, so the downstream apt.dig.net + dig-installer cutover lanes have a released dig-store binary/assets to depend on.

Changes

• CLI binary name — crates/digstore-cli/Cargo.toml [[bin]] name digstore -> dig-store. digs alias, [package] name = digstore-cli, [lib] name = digstore_cli, and every library crate name are UNCHANGED (published/consumed — out of scope by the canonical convention). The shared digstore_cli::run() entrypoint self-reports its argv[0] program name, so dig-store --version reports dig-store 0.14.0 and digs --version reports digs 0.14.0 (verified against the built binaries).
• Release workflow dual-publish (build-binaries.yml) — for ONE transition cycle every asset is published under BOTH the new dig-store-* stem AND the legacy digstore-* stem (bare per-OS binaries + apt .tar.gz), so apt.dig.net + dig-installer stay green until they cut over. --bin digstore --bin digs -> --bin dig-store --bin digs.
• ${DIGS_BIN} substring fix (load-bearing) — the old DIGS_BIN="${DIGS_BIN/digstore/digs}" derived the digs asset name by substring replacement on the primary. After the rename the primary is dig-store, which contains NO digs substring, so the trick is broken. The digs name is now derived INDEPENDENTLY (digs + the OS .exe suffix), guarded by a new shape test.
• Transitional compat symlink — the apt .tar.gz (and the .deb layout via apt) ships a digstore -> dig-store symlink at its root for one cycle, so scripts calling digstore directly keep working (the digs alias already shields interactive users).
• Self-updater (update.rs) — select_binary_asset now prefers the primary dig-store stem over the legacy digstore/digs alias (fixes a latent ambiguity: today's releases already ship both digstore-* and digs-* raw assets, so a naive first-match could self-replace with the alias). extract_digstore_from_targz extracts the real dig-store file (skipping the compat symlink) with a digstore fallback so a dig-store binary can still update from an older, pre-rename tarball.
• release.yml / publish-binary.yml — build + upload the dig-store binary. The dighub S3 hub-worker layout (digstore/<ver>/digstore + latest.json) is UNCHANGED — it is the hub compile-worker's contract, a separate consumer NOT cut over in this lane.
• action.yml — runtime invocations call dig-store; repository: DIG-Network/digstore -> dig-store. cargo install --path crates/digstore-cli unchanged (package name kept).
• crate repository= metadata — digstore-core + dig-client-wasm (+ the wasm pkg assembler) -> github.com/DIG-Network/dig-store.
• SPEC.md / README / runbook / issue templates — binary/product/repo-URL refs -> dig-store; digs alias invocations + historical CHANGELOG untouched; crate names (digstore-*) preserved.

SemVer rationale

MINOR (0.13.5 -> 0.14.0): a user-facing binary rename that stays backwards-compatible via the transitional dual-publish + digstore compat symlink + the digs alias — no consumer breaks.

Verification

• cargo build -p digstore-cli --bins green; dig-store/digs binaries self-report their own names + Usage: lines.
• Full cargo test -p digstore-cli green (46 test-result blocks, 0 failures), incl. new update.rs tests (dual-publish stem preference, dig-store extraction, compat-symlink skip) + new nightly_release_workflow_shape.rs guards (dual-publish both stems, compat symlink, independent digs-name derivation).
• cargo fmt --all -- --check clean; cargo clippy -p digstore-cli --all-targets --all-features -- -D warnings clean.

Blast radius (gitnexus + manual)

invoked_bin_name() (arg0-derived) is consumed only by lib.rs + completion.rs, so the program name flips to dig-store automatically for usage/version/help-json/completions — the only test assertions updated are those dynamic-name checks + the cargo_bin("digstore") -> cargo_bin("dig-store") invocations. In-CLI hardcoded help EXAMPLES (digstore init, ...) were intentionally LEFT as digstore for this transitional release (they remain runnable via the compat symlink); canonicalizing them to the dynamic invoked-name is filed as a follow-up.

Do NOT merge yet

Hand back to the review gate. After merge + a dig-store release (release-first) + pointer bump, dispatch the apt [3a] + dig-installer [3b] cutover lanes.

Refs #703