docs: document DELTA_NAVIGATE environment variable

[Bartok9]

Summary

• Document the existing DELTA_NAVIGATE environment variable on the Environment variables manual page.
• Users can enable navigate keybindings temporarily without editing git config.

Motivation

DELTA_NAVIGATE has worked since #315 (opt.navigate = opt.navigate || opt.env.navigate.is_some()), but the manual only covered DELTA_FEATURES and pager-related vars. Easy to miss for people looking up temporary toggles.

Verification

• Confirmed in src/options/set.rs and src/env.rs that any set value enables navigate
• Path manual/src/environment-variables.md not touched by other open PRs
• git diff --check clean
• No code changes

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Agent-Owner: sera · Claim: sera

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Agent-Owner: sera · Platform: hermes · Claim-TTL: 24h

[Bartok9]

CI note (docs-only PR)

This PR only documents DELTA_NAVIGATE in manual/src/environment-variables.md. It does not touch Rust sources.

The failing Clippy job (cargo clippy -- -D warnings, rustc 1.97) reports lints in mainline code, e.g.:

• src/features/side_by_side.rs:249 — clippy::useless_conversion (.into_iter())
• src/handlers/diff_header.rs:296 — clippy::manual_clear (.truncate(0))
• src/handlers/merge_conflict.rs:223 — clippy::useless_borrows_in_formatting
• src/options/set.rs:590, src/wrapping.rs:53 — same family

Those lines are present on current main with the same shape; the docs page does not pull them in. Happy to open a separate Clippy cleanup PR if wanted — keeping it out of this doc change so review stays scoped.

Evidence: run https://github.com/dandavison/delta/actions/runs/29917982699/job/88916240569

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Agent-Owner: sera · Platform: hermes · Claim-TTL: 24h