docs(static-variables): add prompt-injection threat model and trust tiers

Expands tools/static-variables-and-aliases.mdx with the security framing
that customers building progressive-authentication flows need:

- Lead with the naming distinction between function.parameters
  (LLM-facing JSON schema) and the top-level parameters array
  (server-merged, LLM-invisible). The dashboard surfaces both as
  similarly-named sections, which has bitten customers.
- Document the full Liquid variable bag in three trust tiers:
  Tier 1 server-trusted (customer.*, phoneNumber.*, transport.*,
  call.*, assistant.*, time, assistantOverrides.variableValues set
  at call start), Tier 2 conversation-derived (messages, transcript,
  prompt -- not safe as a security boundary), Tier 3 LLM-derived
  (variableExtractionPlan aliases from non-trusted sources, handoff
  arguments, handoff schema extraction).
- Five common failure modes with bad/good code pairs covering the
  patterns that defeat the security boundary (defining the trusted
  field in function.parameters; body-default leak; system-prompt
  forwarding; unsafe alias chains; mid-call bag mutation).
- Worked example for caller-ID-based progressive authentication.
- Tool-type support matrix, plus an explicit warning that legacy
  assistant.model.functions[] does NOT support static parameters.
- Cross-link to /squads/passing-data-between-assistants for the
  three-approaches handoff guide; flag the squads guide's Approach 1
  (function.parameters on handoff) as not-a-security-boundary for
  signaling-derived values.
- Document the known limitation that destination.assistantOverrides
  .variableValues is not Liquid-resolved at handoff time.
- Dashboard mental model: Parameters (JSON schema editor) vs Static
  Body Fields (key/value rows with Liquid).

Surfaced by Mudflap asking how to do progressive auth without giving
the LLM a path to forward a fake caller-ID.

Skipped: workflow-extraction failure mode (workflows are deprecated).
Skipped: test-writer / code-reviewer (docs-only PR, per write-pr rules).

Author: dhruva-reddy