feat(opencode): add interactive visualize command

[prethiv]

Issue for this PR

Closes #48585

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a /visualize command and opencode visualize CLI with an interactive confirmation step.

Problem: When agents explain complex architectures, distributed flows, or database schemas, they either dump 150+ lines of raw Mermaid/ASCII markup into the terminal or produce long text explanations that are hard to visualize.

Changes made:

1. Added packages/opencode/src/cli/cmd/visualize.ts and registered it in src/index.ts.
2. Uses @clack/prompts to confirm output destination (browser, terminal, ascii, file, or skip) before rendering, preventing unsolicited terminal clutter.
3. Supports targets: context (System Context & Session Runtime pipeline), architecture (layer topology), flow <module> (sequence diagrams), schema <path> (database ERD), deps (package graph), state (drain lifecycle), and git diff inference.
4. When running in CLI mode, it always compiles a self-contained dark-mode HTML artifact with Mermaid.js, pan/zoom controls, and SVG download, presenting a clickable file:/// link to the user.
5. Added custom command .opencode/command/visualize.md, onboarded conventions into AGENTS.md, and added .opencode/command/teach.md.

Why it works: Separates diagram generation from terminal printing behind a selection prompt, while generating a standalone HTML canvas with client-side Mermaid rendering for rich visual inspection without external server dependencies.

How did you verify your code works?

Tested locally using Bun:

1. Ran bun packages/opencode/src/index.ts visualize --help and verified flags (--dest, --format, --output, --yes).
2. Ran bun packages/opencode/src/index.ts visualize context -t ascii -y and verified clean Unicode box-drawing output in terminal.
3. Ran bun packages/opencode/src/index.ts visualize context -t browser -y and verified it created the .html artifact in .opencode/artifacts/ and opened it in the default browser.
4. Ran automated unit tests:

   bun test packages/opencode/test/cli/visualize.test.ts # 7/7 passed
   bun test test/cli/help/help-snapshots.test.ts         # 34/34 snapshots passed
   bun --cwd packages/opencode typecheck               # 0 errors

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[prethiv]

Manual testing screenshots of SAGA Pattern.

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