session_start hook cannot inject context into the model (observer-only)

Problem

jcode's session_start lifecycle hook is observer-only: per docs/HOOKS.md, observer hooks are "spawned detached, fire-and-forget" and their stdout is discarded. The only synchronous hook, pre_tool, can block a tool call (exit 2) but cannot inject prompt text either.

As a result, there is no way for an external hook to add per-session context into the model's prompt in jcode.

Claude Code, by contrast, lets a SessionStart hook return additionalContext that is injected into the conversation. Tools built on that pattern (e.g. the caveman skill's always-on activation hook, or any "inject git state / rotating rules / mode flag each session" hook) work in Claude Code but have no jcode equivalent.

Today the only jcode mechanism for per-session instructions is a static ~/AGENTS.md, which cannot be computed or made conditional per session.

Impact

• No dynamic/computed per-session context injection (git state, pending reviews, mode flags, rotating guidance).
• Cross-harness tools that rely on hook-injected context silently degrade to "static file only" on jcode.

Possible solution

Add an injecting hook path, opt-in and back-compat:

• Option A: let a new session_context hook's stdout (capped, e.g. 16 KB like JCODE_HOOK_PAYLOAD) be appended to the session as system/context injection.
• Option B: extend session_start with an opt-in mode where stdout is captured and injected, leaving the default observer behavior unchanged.

Keep the existing observer session_start semantics intact so nothing breaks; gate the new behavior behind a distinct config key.

Environment

• jcode (docs/HOOKS.md, master)
• Observed while wiring an always-on caveman-style context injection across Claude Code + jcode; only Claude Code could do it via hook.

Start with docs/HOOKS.md to understand the current session_start observer semantics and the JCODE_HOOK_PAYLOAD size convention. Trace the session_start hook entry point, then implement an opt-in captured-output path that injects bounded context while preserving the default detached behavior; done means dynamic session context is available without breaking existing observer hooks.