Add editing for existing custom fields

[m11y]

Summary

• Add rbw edit --field=<name> for updating one existing custom field on login, secure note, card, and identity entries from piped stdin or an interactive editor.
• Reject empty values, SSH key entries, linked fields, and invalid boolean values; on a revision conflict, rebase only when the target field's plaintext did not change.
• Add rbw get --list-custom-fields, Fish completion, documentation, and focused tests for field lookup, help stripping, validation, and conflict comparison.

This branch is stacked on #371. Until that PR merges, GitHub's base comparison also shows its safety changes; the feature-only change is commit d21dc8f and touches five files.

Test plan

• cargo test --all-features
• cargo clippy --bin rbw --lib -- -D warnings
• cargo fmt --check
• fish -n src/bin/rbw/completion/rbw.fish
• Pipe a value into rbw edit --field=<existing> <entry> and verify it with rbw get --field=<existing>
• Verify a failed pipe edit does not create an unsaved-edit file
• Verify concurrent edits to another field rebase while edits to the same field conflict
• Verify keyed items remain decryptable and SSH key / linked field edits are rejected

Depends on: #371
CI's lint job is red here for reasons that predate this branch: stable clippy 1.98 fails --all-targets --all-features -- -Dwarnings on main itself, at eight sites in code this PR does not touch. #373 fixes those. That job then fails again at cargo deny check on RustSec advisories in Cargo.lock (bytes, quick-xml, rand, rustls-webpki, yanked spin), which is also independent of this branch and needs a dependency refresh. cargo clippy --bin rbw --lib, cargo fmt --check, and cargo test --all-features are clean on this branch.

Related: #364