## Summary

This PR extends the `always_allow` tool permission patterns to work with
Nushell, Elvish, and Rc shells. Previously, these shells were
incorrectly excluded because they don't use `&&`/`||` operators for
command chaining. However, brush-parser can safely parse their command
syntax since they all use `;` for sequential execution.

## Changes

- Add `ShellKind::Nushell`, `ShellKind::Elvish`, and `ShellKind::Rc` to
`supports_posix_chaining()`
- Split `ShellKind::Unknown` into `ShellKind::UnknownWindows` and
`ShellKind::UnknownUnix` to preserve platform-specific fallback behavior
while still denying `always_allow` patterns for unrecognized shells
- Add comprehensive tests for the new shell support
- Clarify documentation about shell compatibility

## Shell Notes

- **Nushell**: Uses `;` for sequential execution. The `and`/`or`
keywords are boolean operators on values, not command chaining.
- **Elvish**: Uses `;` to separate pipelines. Does not have `&&` or `||`
operators. Its `and`/`or` are special commands operating on values.
- **Rc (Plan 9)**: Uses `;` for sequential execution and `|` for piping.
Does not have `&&`/`||` operators.

## Security

Unknown shells still return `false` from `supports_posix_chaining()`, so
`always_allow` patterns are denied for safety when we can't verify the
shell's syntax.

(No release notes because granular tool permissions are still
feature-flagged.)

Release Notes:

- N/A

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Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>