Support Zed review task and snippet fields

[ikma-ricon]

Summary

• add optional Zed task reveal, shell, shell-argument, and environment fields
• add checked editor::InsertSnippet keymap bindings with conflict and force behavior
• preserve the existing minimal task shape when new fields are omitted
• validate policy enums and environment entries before writing
• document the concrete inline-review workflow

Design choices

• a shell program without arguments uses Zed's simple { "program": ... } form
• shell arguments use shell.with_arguments
• same-label task upsert continues replacing the whole task, including its environment
• snippet support remains limited to the proven inline editor::InsertSnippet action

Validation

• mise run test (34 tests)
• mise run doctor
• codebase lint (9 rules)
• readme build --check
• Bash syntax checks for changed tasks
• git diff --check

Closes #1