Docs - Document `onInit` / `postAttach` semantics (typed into PTY, not exec'd)

[eminwux]

Add a section to docs/site/concepts/profiles.md explaining:

• onInit and postAttach steps are typed into the shell stdin, not exec'd as subprocesses
• Env key/value pairs are prefixed inline (FOO="bar" script), not exported
• Foreground vs background behavior — if onInit launches a long-lived process, terminal state transitions to Ready immediately after writing the line
• Caveats: multi-line scripts, here-docs, env scoping

Why: Users currently expect shell-level semantics (env exported to parent shell, script completes before prompt). The real behavior is in internal/terminal/terminalrunner/lifecycle.go:259-282 but undocumented.

From gap-analysis.md §7 (P2).