feat(source): follow the remote default branch when no ref is pinned

[srnnkls]

What

A git source with no branch/tag/rev resolved to a hardcoded main. A repository whose default branch is master or trunk failed to resolve and forced an explicit branch =. Resolution now defers to the repository's own default branch.

How

• New Refspec::Default for the no-ref case; refspec() / intrinsic_refspec() return it instead of Branch("main").
• Resolved by peeling the bare mirror's HEAD, which gix sets from the remote's advertised default at clone. Incremental fetches advance that branch's ref, so peeling HEAD always lands on the current tip.
• Encoded as a stable default discriminator (network-independent), so the lock stays reproducible and encode_ref / Display round-trip without a branch name. A no-ref source records resolved = "default" with no ref discriminator.

Behavior

• master/trunk/main repos all just work with no config.
• phora update re-resolves the default branch to its current tip, exactly as it would track an explicit branch.
• The pinned commit in the lock is unchanged by all this; only update re-resolves.

Known edge

HEAD is written once at clone and not refreshed if upstream renames its default branch — the stale symref resolves to the old branch until the disposable cache is re-cloned. The locked commit is unaffected. Pin an explicit branch to track a rename without a re-clone. Upgrade path: capture the HEAD symref from the fetch handshake and rewrite mirror HEAD each fetch.

Tests

• resolve_default_follows_remote_default_branch_when_not_main — a trunk-default repo (no main) resolves via Default.
• resolve_default_survives_an_incremental_fetch — tracks the tip across an incremental fetch.
• Updated the two unit tests that pinned the old main default.
• Re-validated on the main base after rebase: 1267 lib + all integration binaries + 7 scrut suites + clippy (pedantic, -D warnings) + rustfmt.

Docs

README documents the user-facing default. The GUIDE.md walkthrough for this change lands with the docs branch (#11) — GUIDE.md does not exist on main, so it cannot ride in this PR.