feat: allow launchers to override the resume hint

[alupao]

Problem

Tools that launch Claudex cannot provide their own stable resume command.
Claude Code prints claude --resume ..., and Claudex then appends a second,
provider-specific claudex run ... command containing transient launch
arguments. Wrapper applications therefore expose implementation details and
cannot direct users back through their own session abstraction.

Proposed behavior

Support an optional CLAUDEX_RESUME_HINT environment variable for interactive
Unix sessions:

• replace the detected native claude --resume ... command with the exact
  one-line value;
• omit the additional Claudex resume footer when the override is active;
• activate the existing PTY proxy so replacement also works when hyperlinks
  are disabled;
• ignore empty values and values containing terminal control characters;
• preserve current behavior completely when the variable is unset.

Example:

CLAUDEX_RESUME_HINT="my-tool resume session-123" claudex run profile

This keeps the feature generic: Claudex does not need to know anything about
the wrapper's session format.