Add an external_directory permission for directory grants #157

Status

Speculative. Depends on a future TUI (or other interactive frontend)
being added to this project. ReloadedCode is headless today.

Problem

Agents get file access through a PathResolver
(AllowedPathResolver, AllowedGlobResolver) built once, at agent
construction time.

The permission model (reloaded_code_core::permissions::Ruleset) already gates
tools (read, write, edit, bash, task, ...) with default-deny,
last-match-wins allow/deny rules.

A TUI would let a user point an agent at a new folder during a
session, for example "also work in ~/other-project". Today there is
no permission key that governs which directories a session may ever
add.

Current behavior

• permission: in agent frontmatter has no external_directory key.
• Path resolvers fix their allowed directories at construction time.
• Nothing in reloaded-code-core gates a runtime directory grant.

Expected behavior

Add external_directory as a permission key, evaluated with the same
Ruleset used for every other permission:

permission:
  external_directory:
    "~/other-project/**": allow
    "/etc/**": deny
    "**": deny

• Default is deny, matching every other unlisted permission today.
• Patterns match against the candidate directory path (or a path
  inside it), using the existing glob/wildcard pattern syntax.
• Evaluation is last-match-wins, same as read/write/task rules.

Evidence

• src/reloaded-code-core/src/permissions.rs - Ruleset,
  PermissionAction, last-match-wins evaluation.
• src/reloaded-code-core/src/path/allowed.rs - AllowedPathResolver.
• src/reloaded-code-core/src/path/allowed_glob/ - AllowedGlobResolver,
  GlobPolicy.
• src/reloaded-code-agents/README.md (permission table, pattern
  rules for read/write/edit/glob/grep/bash/task).
• README.MD ("No TUI. Embed it anywhere.").