Pipe terminal-profile `args` (and decide on `env`) to the agent host

[anthonykim1]

Follow-up to #313160 / #313679.

#313679 made the agent host respect the user's preferred shell executable by piping terminal.integrated.agentHostProfile.<os> (with fallback to defaultProfile.<os>) down to the agent host as AgentHostConfigKey.DefaultShell. Only the path is honored today — args and env from the profile are silently dropped.

That means a user with, say:

"terminal.integrated.profiles.windows": {
    "Git Bash": { "source": "Git Bash", "args": ["--login", "-i"] }
}

…gets bash.exe spawned in the agent host without --login -i, so their ~/.bash_profile doesn't run, PATH tweaks/aliases/exports the user expects don't apply. Same story for --noprofile, custom --rcfile, oh-my-zsh-style -i, etc. macOS bash/zsh --login is the only argument added automatically today (in AgentHostTerminalManager.createTerminal).

Reasons we deferred this on #313679:

1. Args splicing has order-sensitivity with shell-integration injection. getShellIntegrationInjection overwrites shellArgs with its own newArgs (e.g. --init-file … for bash, -NoProfile -Command "& …" for pwsh). Mixing user args needs per-shell-family rules (before the integration script for bash, before -Command for pwsh, etc.).
2. Env merging needs an explicit precedence policy with the agent host's curated env. We already set LC_ALL=C.UTF-8, PAGER='', GIT_TERMINAL_PROMPT=0, DEBIAN_FRONTEND=noninteractive, VSCODE_PREVENT_SHELL_HISTORY, plus shell integration's envMixin. User-profile env (e.g. LC_ALL=en_US.UTF-8, PAGER=less, custom PROMPT_COMMAND) can break agent output parsing if merged naively.
3. The single-string defaultShell wire shape stays clean for headless code agent-host and SSH/tunnel agent hosts, where workbench profile semantics don't apply.

What this issue should cover:

• Extend the AHP root-config schema to carry args (AgentHostConfigKey.DefaultShellArgs as a sibling string-array key, keeping DefaultShell as-is for back-compat).
• Workbench bridge in AgentHostTerminalContribution._pushDefaultShell resolves and pushes both keys.
• AgentHostTerminalManager.createTerminal reads the configured args from root config and splices them in correctly relative to shell-integration injection.
• Decide on env policy. Options: (a) ignore profile env entirely (current behavior), (b) whitelist a small set, (c) merge with agent-host curated env winning. Probably (a) or (b).
• Update setting descriptions and AgentHostConfigKey.DefaultShell JSDoc to remove the "args/env are ignored" caveat (or update it to reflect the new behavior).
• Tests for args splicing per shell family.

/cc #313160 #313679