fix(brand): give cA2A its own icon and align to brand palette

cA2A and cMCP were shipping byte-identical docs/assets/icon.svg files
(md5 5ff302ae5beb0820351a5a958ca52c14). Two separate projects, one padlock
mark, in both the docs header and the docs favicon.

Replace it with a glyph specific to what cA2A does: two nodes joined by an
arc with a red arrowhead, reading as a delegation hop where authority narrows
in the direction of travel. Distinct from the cMCP padlock and from the Agent
Manifest node triangle.

Also moves off the MkDocs Material default gradient onto the AgenTrust palette
(navy #15294B to #1B5EA0, red accent #B91C1C) defined in brand/BRAND.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: imran-siddique