rbw edit: support editing a single field (--field)

[NanShanFish]

rbw edit: support editing a single field (--field)

Feature request

rbw edit currently only edits an entry's password + notes (the first line of
the editor buffer is the password, the remainder after a blank line is notes).
There is no way to change the username, entry name, TOTP secret, URI list, or a
custom field from the CLI.

rbw get already supports --field to display a single field; this feature
adds the analogous rbw edit --field to edit one.

Proposal

rbw edit --field <field> <entry> opens the editor with the field's current
plaintext value (uris shown one per line) and saves the edited value back.

Supported fields:

• username — Login entries
• name — entry name (any type)
• notes — any type
• password — Login entries (records the old password into history)
• totp — Login entries
• uris — Login entries, one URI per line (existing per-URI match types are
  preserved for unchanged URIs)
• any other string — matches a custom field by name substring (same semantics
  as rbw get --field)

Empty input clears the field.

Non-interactive usage works too (piped stdin), consistent with the existing
rbw edit behavior.

Implementation notes

This feature builds on the item-key edit fix (editing keyed entries must
encrypt with the entry's individual key and preserve the key field), so the
feature branch is based on that fix.