Mattermost fullstack connection atlas planning docs added

BLUF: Added an Obsidian vault planning layer for mapping Mattermost webapp <-> Go server connections at route/function level, intended for a manual opencode session. No app runtime code changed.

Files created:

• /home/tlswk/projects/mattermost/mattermost/webapp/docs/webapp-vault/50-fullstack-map/README.md
• /home/tlswk/projects/mattermost/mattermost/webapp/docs/webapp-vault/50-fullstack-map/00-visualization-and-data-plan.md
• /home/tlswk/projects/mattermost/mattermost/webapp/docs/webapp-vault/50-fullstack-map/01-opencode-work-order.md
• /home/tlswk/projects/mattermost/mattermost/webapp/docs/webapp-vault/50-fullstack-map/02-opencode-prompt.md

Files updated:

• /home/tlswk/projects/mattermost/mattermost/webapp/docs/webapp-vault/README.md
• /home/tlswk/projects/mattermost/mattermost/webapp/docs/webapp-vault/STATUS.md

Key decision:

• Use /home/tlswk/projects/mattermost/mattermost as the canonical repo because the vault lives there. /home/tlswk/mattermost-src remains reference-only; it is a separate checkout at a different commit.
• Do not ask opencode to draw only a big diagram. First generate fullstack-connections.json and Markdown matrices, then generate Obsidian Canvas and Excalidraw-style overview from that evidence.
• Manual opencode session only; no autonomous worker fleet, scheduler, cron, or app behavior changes.

Verification:

• Markdown fence balance check passed for new fullstack-map docs plus README/STATUS.
• git status confirms docs are still under untracked webapp/docs/ as expected from the existing vault state.

Next action:

• Feed docs/webapp-vault/50-fullstack-map/02-opencode-prompt.md to a manual opencode session from /home/tlswk/projects/mattermost/mattermost/webapp.