docs: comprehensive plumbing and porcelain reference update

Plumbing doc: - read-commit, read-snapshot: document --format text one-liner output - commit-tree: document --format text (bare commit ID), add security note about snapshot/parent ID validation - update-ref: document --format text (silent on success), add security note about branch-name validation preventing path traversal - unpack-objects: document --format text human summary - ls-remote: update from --json/-j to --format json|text (API change) - commit-graph: note 100 000-commit cap on --ancestry-path BFS - Fix composability example to use ls-remote --format text

Porcelain doc: - Add complete --format json flag table and JSON payload schema to every command: commit, status, log, diff, show, branch, checkout, merge, rebase, reset, revert, cherry-pick, stash, tag, blame, reflog, gc, archive, bisect, worktree, describe, shortlog, verify, snapshot, bundle, content-grep, whoami, config - Fix whoami JSON: token_set is boolean; add capabilities field - Fix diff JSON: field is total_changes, not ops - Add rebase JSON: two distinct shapes (squash_rebase vs rebase) - Add config show JSON: full schema with user/hub/remotes/domain sections, blocked namespaces documented - Add flag tables with short flags for all commands - Expand composability patterns section with 3 new examples