Converge the intent corpus under context/intent

[schickling-assistant]

Why

Issue #8 exposes two conflicting corpus roots: the checker treats intent/ as the corpus while exported review workspaces operate from the repository root. This converges both seams on the current main contract.

What

• Move the root corpus from intent/ to canonical context/intent/ and update owned links, fixtures, CLI defaults/help, CI, and Nix checks.
• Preserve PR Use Intent as the canonical system name #10's canonical vocabulary, AXE.INTENT-R IDs, schemas, APIs, tests, and CI without compatibility aliases.
• Semantically carry forward PR Fix the three extraction regressions: false-green whole-repo check, review workspace root, displayed defaults #6's still-needed defaults, repo-root decision fallback, exported-workspace sentinel, and regression coverage on top of current main.

Verification

• RED (parent): a malformed repo-root decision produced no matching meta-decision-shape diagnostics; GREEN (candidate): the equivalent context/intent record produced four matching diagnostics.
• RED (parent): an exported no-.git review invoked the checker with --cwd=<export>/intent; GREEN (candidate): it invoked the checker with the exported repository root. Both review invocations exited 0, isolating the path-selection seam.
• Exact candidate: cargo fmt --check, cargo test --locked (10 unit, 12 integration, 0 doc), cargo build --locked, cargo clippy --locked --all-targets -- -D warnings, and nix flake check -L all pass. Strict corpus validation reports 0 diagnostics; the graph contains 246 nodes and 215 edges.

Closes #8
Supersedes #6