feat: implement advisor subagent

[loss-and-quick]

Issue for this PR

Closes #

Type of change

• New feature

What does this PR do?

Implements advisor as a native subagent for strategic architecture advice, security review, and complex trade-offs.

How it works:

• Registered conditionally when OPENCODE_EXPERIMENTAL_ADVISOR=1 or experimental.advisor: true
• Uses standard task tool routing: @advisor <question> triggers task tool with subagent_type: "advisor"
• Read-only permissions: grep, glob, list, read, bash, webfetch, websearch (no write/edit)

Why this approach:

• Consistent with other subagents (explore, general)

How did you verify your code works?

# Verify advisor appears in agent list with flag
OPENCODE_EXPERIMENTAL_ADVISOR=1 ./result/bin/opencode agent list | grep advisor

# Verify advisor absent without flag
./result/bin/opencode agent list | grep advisor

# Test advisor invocation
OPENCODE_EXPERIMENTAL_ADVISOR=1 ./result/bin/opencode run "@advisor How to organize modular architecture?"

Nix build succeeds: nix build .#opencode

Screenshots / recordings

N/A - CLI feature

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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