docs: sync website docs for v0.6.1 release

[ajianaz]

Summary

Brings the website docs in sync with the v0.6.1 patch bundle (#316 / #312 / #313 / #314) before cutting the release tag.

Changes

docs/changelog.md

Was stuck at v0.2.0 — now a full sync from root CHANGELOG.md (includes v0.4.x, v0.5.x, v0.6.0, and v0.6.1 with all compare links at the bottom).

docs/installation.md

• Update pinned-version example: CORA_VERSION=v0.6.0 → v0.6.1.
• Add Check for stale copies on PATH subsection with which -a cora snippet and cross-link to docs: install instructions don't warn about multiple distribution channels causing PATH conflicts #314.
• Add macOS: Killed: 9 on launch? subsection documenting the Gatekeeper quarantine workaround (xattr -dr com.apple.quarantine / com.apple.provenance) from bug: macOS binary release killed by Gatekeeper quarantine (com.apple.quarantine/provenance) #313.
• Replace placeholder cora 0.x.x in the verify snippet with cora 0.6.1.

docs/cli-reference.md

Document the two new cora scan flags from #316:

• --batch-files <N> — max files per LLM batch (default 20)
• --no-continue-on-batch-error — abort instead of skip-and-continue

docs/usage.md

Add a tip about lowering --batch-files for large scans that hit provider token limits / rate-limits, and the non-fatal batch behavior (#316).

README.md

Update pinned-version example to v0.6.1.

Tests

Docs-only. cargo build, cargo clippy -- -D warnings, cargo fmt --check, and cargo test (564 + 16 + 6) all still pass to keep the release branch green.

Checklist

• All version references updated to 0.6.1
• Website changelog synced with root CHANGELOG
• New flags documented in CLI reference
• macOS Gatekeeper + multi-channel install gotchas documented
• cargo build / clippy / fmt / test clean