Reposition docs around custody serving the court

[kridaydave]

PR made by x-preview-f from opencode on behalf of Kriday.

Oot now keeps its own history, and the docs still said it never touches a repository. This PR rewrites the story so custody leads: the court governs changes, the store exists to serve it, git stays the interchange format.

Summary:

• README intro tells the custody story (adapted from the approved seed); the false 'does not try to replace Git / never owns the repository' lines are gone; tagline untouched
• 'Where Oot sits' bullet 1 replaced: Oot keeps its own store, git is the export target, unfiltered export reproduces byte-identical SHAs
• 'How Oot thinks' gains a Store bullet (native history, parents as change ids)
• Status checklist gains six shipped items (store, import, record, log/status, byte-identical export, visibility-filtered export) plus an honest-limits footnote pointing at TODO deliberate cuts
• Showcase docket swapped for real fixture output (exits 1 on purpose); embargo distribution demoted to roadmap
• Try-it adds the native workflow (init/status/record/log/export), all commands verified against the built binary
• Someday became Roadmap: working-copy update, store-to-court adjudication, per-subtree signature reuse, gc join intent scoring and embargo distribution
• CONTRIBUTING: two-way bridge framing, new 'Store & exporter contributors' audience, build order lists storage/exporter as DONE, dev setup gains a five-command walkthrough
• oot-vision-report.html gets a SUPERSEDED banner linking TODO.md, body untouched
• visibility.toml comments document both consumers (adjudicate cloaks + exits nonzero; export withholds/rebuilds/refuses under embargo, decisions logged)
• Cargo.toml description moves to 'Repos track lines. Oot governs changes...'
• TODO.md night-run box for PR-O2 flipped to [x]

Calls I made:

• Verified before writing: ran every documented command against the release binary. The spec guessed the mock invocation 'exits 2'; reality is exit 1 (CLOAKED). Exit 2 is what you get from bare adjudicate --change with no snapshots. Documented exit 1.
• Swapped the fake ADJUDICATED docket for real captured output rather than labeling the old one illustrative.
• Fixed two adjacent lines outside the letter of the edit list because custody made them false: 'No branch name, no commit message' in How-Oot-thinks, and the Contribute lead-in about primitives Oot 'sits above'.
• Left the clap about string in src/main.rs alone since src/ is off limits on this branch, so binary help and crate description briefly diverge.
• Inserted Store & exporter as item 2 in CONTRIBUTING's build order and renumbered the rest.
• Kept pre-existing em/en dashes inside the vision report body (banner-only ownership); none added anywhere else.