[Markdown] create documents in workspace

[George Ng (GeorgeNgMsft)]

Summary

This PR fixes document creation and separates open/create semantics

Layer 1 (bottom) of the replacement stack for #2940.

This layer adds only core workspace-safe Markdown document creation:

• propagates the host-authorized working directory through action context and agent RPC
• creates Markdown files under that workspace, including nested relative paths
• preserves requested initial content
• rejects traversal, absolute/drive-qualified, out-of-workspace, and symlink-escape paths
• avoids constructing the Markdown model for create/open actions
• passes the CLI request working directory into dispatch

Deferred to upper layers

• document update/edit semantics
• revision and binding-token conflict checks
• streaming update persistence
• browser-view persistence and synchronization
• reopening editor bindings
• loopback view-server changes
• the unrelated aiclient runtimeConfig/OpenAI change

Validation

• pnpm run prettier:changed:fix / pnpm run prettier:changed (pass)
• lint ratchet (pass)
• complexity ratchet (pass)
• circular-dependency ratchet (pass)
• debt/skipped-test gate (pass)
• direct path-policy smoke covering nested creation, traversal rejection, canonical root resolution, and junction escape rejection (pass)
• affected TypeScript build and Jest suites were attempted but environment-blocked: worktree setup could not fetch locked public packages baseline-browser-mapping@2.11.19 and uuid@14.0.2 because Azure Artifacts returned 401, leaving workspace package links incomplete; the resulting compiler errors were missing-module diagnostics