docs(static-variables): recover orphaned PR #1035 + clarify deterministic ≠ invisible

Two combined goals -- the previous PR #1035 was orphaned by an inverted
merge order (parent #1033 squash-merged to main 25 seconds before child
#1035 squash-merged into the parent's branch, so #1035's content was
applied to a branch that was already obsolete and never propagated to
main). This restores that hardening AND fixes wording the user flagged
in review of the original page.

Recovers from orphaned commit 3bbefdb the full PR #1035 hardening
work: trust tiers (Tier 1 server-trusted, Tier 2 conversation-derived,
Tier 3 LLM-derived), five common failure modes, prompt-injection
threat-model framing, dashboard mental model, handoff-data-forwarding
section, legacy assistant.model.functions[] footgun warning, the
'Static parameters as a security boundary' section, and the worked
caller-ID example.

Wording fixes applied on top of that recovery, eliminating language
that conflated two different guarantees -- (1) static parameters are
truly LLM-invisible (server-merged into the request body, never in
the schema sent to the model) vs. (2) variableExtractionPlan aliases
chain values across tools deterministically but the source response
IS in the LLM's context (it was added to conversation history as a
role: tool message). Specific changes:

- Subtitle (line 3): split the two guarantees, no longer claims both
  are 'without LLM involvement'
- Intro bullet (line 19): 'deterministically -- the next tool gets
  the correct value regardless of how the LLM behaves between calls'
- Deterministic tool chaining intro (line 457): rewritten + new
  Warning callout explicitly stating 'Deterministic does not mean
  invisible' with the specific code-level claim that Tool A's
  response is added to the LLM's role:tool message history
- Tips section: new bullet 'Aliases are a determinism primitive,
  not an invisibility primitive', flagging that hiding values from
  the model requires the tool server to omit them from the response
  body in the first place

Skipped: test-writer / code-reviewer (docs-only PR).

Author: dhruva-reddy