feat(bash): load user shell RC files for alias support

[aryasaatvik]

Summary

• Adds Shell.execute() unified function for shell command execution with RC file loading support
• Bash tool now loads user shell RC files (.bashrc, .zshrc, etc.) enabling access to aliases
• Refactors shell escape (! prefix) to use the same shared execution logic

Changes

src/shell/shell.ts

• Added Shell.execute() function with options for loadRcFiles, timeout, abort, onOutput
• Added getInvocationArgs() helper for shell-specific invocation patterns (zsh, bash, fish, nu, cmd, powershell)

src/tool/bash.ts

• Replaced inline spawn logic with Shell.execute({ loadRcFiles: true })
• Bash tool now has access to user aliases and shell functions
• Added TERM: "dumb" for cleaner output

src/session/prompt.ts

• Refactored shell() function to use Shell.execute()
• Removed duplicated spawn logic

Behavior Changes

Feature	Before	After
Bash tool RC files	Not loaded	Loaded (aliases work)
Bash tool TERM	Inherited	"dumb"
Code duplication	Separate implementations	Shared Shell.execute()