DOCUMENTATION: Add interactive solution dependency graph

[cjdutoit]

Problem

There is no at-a-glance picture of how this solution fits together. Working
out how a PDS search actually reaches NHS Digital — client → processing /
orchestration → foundation → broker → HTTP — means opening a dozen files, and
the parts that are not wired up (unused packages, unused constructor
dependencies, dead types) are invisible until someone trips over them.

Proposal

Add an interactive dependency graph under Documentation/DependencyGraph,
matching the one already added to NHSISL/LondonFhirService
(https://github.com/NHSISL/LondonFhirService/pull/176):

• A single self-contained index.html — no build step, no npm, no runtime
  dependencies — reading all data from a sibling graph-data.js.
• Two views behind one toggle: single copy (every component once, all
  consumers converging on it) and per consumer (dependencies duplicated
  per caller, each copy showing only the rows that caller uses).
• Click a component or a single method row to trace its full upstream and
  downstream slice.
• Published to GitHub Pages so it can be linked from a review or a ticket.
• A /update-dependency-graph skill so the snapshot can be refreshed when the
  code moves, rather than rotting.

Scope

Documentation and CI only — no production code changes.

• Documentation/DependencyGraph/ — index.html, graph-data.js, README.md
• .github/workflows/pages.yml — publishes the folder to GitHub Pages
• .claude/skills/update-dependency-graph/SKILL.md — refresh procedure

Follow-up

The scan surfaces several things worth deciding on separately (they are
recorded in the graph README, not changed here): ApiPlatformClientFacade is
dead code, PdsOrchestrationService injects a token broker it never calls,
and the three ISL.Providers.PDS.* package references are not used by any
source file.

GitHub Pages will need enabling once under Settings → Pages with the
source set to GitHub Actions before the deploy job can succeed.

[cjdutoit]

Raised as #25 — branch users/cjdutoit/documentation-dependency-graph.