* feat: isolate new-tab agent navigation from origin tab
Add origin-aware navigation isolation so the agent never navigates
away from the new-tab chat UI. This is a two-layer defense:
1. Prompt adaptation: When origin is 'newtab', the system prompt's
execution and tool-selection sections are rewritten to prohibit
navigating the active tab and default all lookups to new_page.
2. Tool-level guards: navigate_page and close_page reject attempts
to act on the origin tab when in newtab mode, returning an error
that teaches the agent to self-correct.
The client now sends an `origin` field ('sidepanel' | 'newtab')
instead of injecting a soft NEWTAB_SYSTEM_PROMPT that LLMs could
ignore. Backwards compatible — defaults to 'sidepanel'.
Closes TKT-592, addresses TKT-564
* test: add newtab origin navigation guard tests
- 14 new prompt tests verifying the system prompt adapts correctly
for newtab vs sidepanel origin (execution rules, tool selection table,
absence of conflicting single-tab guidance)
- 6 new integration tests for navigate_page and close_page guards:
rejects origin tab in newtab mode, allows non-origin tabs, allows
all tabs in sidepanel mode, backwards compatible with no session
