feat(shell): add shell.env to inject environment variables

[roramirez]

Summary

Adds a TOML-only shell.env configuration list that injects environment variables into every spawned shell. Format: env = [["VAR", "value"], ["VAR2", "value2"]]. This is intentionally not editable in the in-app TUI config panel, but is preserved across TUI saves.

Changes

• src/config/mod.rs: add #[serde(default)] pub env: Vec<(String, String)> to ShellConfig (empty vec by default).
• src/pty/session.rs: spawn_with_shell gains an env: &[(String, String)] parameter; each pair is applied via cmd.env(k, v) after TERM. spawn passes &[].
• src/pane_ops.rs: pass &self.state.config.shell.env when spawning a pane shell.
• src/config/tui_config.rs: carry shell.env through the config-panel round-trip (non-editable) so saving from the TUI does not wipe it.
• assets/config.toml, README.md, doc/SPEC.md: documented commented example and config table entry; noted it is TOML-only.
• CHANGELOG.md: ### Added entry.
• Tests: pty env-injection test, config round-trip/deserialize/default tests; updated all spawn_with_shell callers.

How to test

• cargo test — includes env_vars_are_injected_into_shell (spawns /bin/sh, injects MMTERM_TEST_ENV=hello, asserts output) and shell_env_* config tests.
• cargo clippy --locked -- -D warnings and cargo fmt --check are clean.
• Manual: add env = [["EDITOR", "nvim"]] under [shell] in config.toml, launch mmterm, run echo $EDITOR in a pane.