feat: port apfel demos into aichat

[renato-umeton]

Summary

• Add 7 built-in roles ported from apfel demos: oneliner, explain, naming, wtd, port, gitsum, mac-narrator
• Add bash/zsh shell functions (scripts/shell-integration/demos.{bash,zsh}) that gather context and pipe to the appropriate role, plus cmd as an alias for aichat -e
• Skip cmd as a role since it duplicates the existing --execute / %shell% functionality

Usage

Roles work directly:

aichat -r oneliner "sum column 3 of a CSV"
aichat -r naming "function that retries HTTP requests"
aichat -r explain "curl -sSL -o /dev/null -w '%{http_code}'"

Shell functions (source demos.bash or demos.zsh) add context-gathering wrappers:

cmd "find all .log files modified today"   # alias for aichat -e
wtd ~/some/project                         # gathers dir info automatically
port 3000                                  # runs lsof, pipes to role
gitsum 20                                  # gathers git log, pipes to role
mac-narrator                               # gathers system stats, pipes to role

Test plan

• cargo test --all -- 21 tests pass
• cargo clippy --all --all-targets -- -D warnings -- clean
• cargo fmt --all --check -- clean
• Manual: aichat --list-roles shows new roles
• Manual: aichat -r naming "function that retries HTTP" returns naming suggestions
• Manual: source demos.zsh and run gitsum in a git repo

Closes #1504