Propagate direnv/mise/nix environments to agent sessions (login shells)

[Ecko95]

Ported from upstream pingdotgg#523.

Problem

Agent sessions and terminals spawn subshells that are not login shells, so per-project environment managers (direnv, mise, nix flakes) never hook in. Agents end up missing the project's toolchain/env vars, while the same commands work in the user's own terminal.

Proposed fix

Smallest viable change: spawn agent/provider subshells as login shells (bash -l / $SHELL -l) or source the direnv hook before exec, so .envrc / mise / nix develop environments apply.

Notes

• Upstream workarounds users resort to: wrapping commands in bash -l -c or nix develop -c manually.
• Interacts with our server-side env injection (deterministic session port env, feat(server): inject per-session GITS_PORT into agent sessions and terminals #124) — make sure injected vars survive the login-shell profile.
• Check both provider adapter spawns and integrated terminal spawns.

Acceptance

A project with an .envrc (direnv allow'd) gets its env vars inside an agent session's shell commands without manual wrapping.