docs: update sssc-planning agent-overview for consolidated instructions and new skill

[github-actions]

Component: Documentation

Bug Description

docs/agents/sssc-planning/agent-overview.md is stale after PR #1950 (commit 2e9608cc648b) refactored the SSSC Planner instruction files.

The documentation still describes the old multi-file instruction architecture that no longer exists:

1. Architecture diagram (lines 35–48) lists six separate instruction nodes that were removed:

   • sssc-identity, sssc-assessment, sssc-standards, sssc-gap-analysis, sssc-backlog, sssc-handoff

2. Line 50 states: "The agent follows six SSSC-specific instruction files, each scoped to a specific phase concern."

3. Related Files table (line 131) does not include the new supply-chain-security skill that the planner now depends on for standards and taxonomy references.

Expected Behavior

After the update, the documentation should accurately reflect:

• The architecture diagram should show a single consolidated instruction node (sssc-planner) instead of the six removed files.
• The prose should describe one consolidated sssc-planner.instructions.md file (not six separate files).
• The Related Files table should include a row for the supply-chain-security skill at .github/skills/security/supply-chain-security/.

Steps to Reproduce

See the code change that made this documentation stale:

• Commit: 2e9608cc648b3baf336f7559c6c9506f36761afc
• PR: feat(security): extract supply-chain-security skill and thin SSSC Planner #1950 — feat(security): extract supply-chain-security skill and thin SSSC Planner

The six removed files were:

• .github/instructions/security/sssc-identity.instructions.md
• .github/instructions/security/sssc-assessment.instructions.md
• .github/instructions/security/sssc-standards.instructions.md
• .github/instructions/security/sssc-gap-analysis.instructions.md
• .github/instructions/security/sssc-backlog.instructions.md
• .github/instructions/security/sssc-handoff.instructions.md

The replacement is:

• .github/instructions/security/sssc-planner.instructions.md (consolidated, single file)

The new skill added:

• .github/skills/security/supply-chain-security/ (with SKILL.md and references/)

Additional Context

• Stale documentation file: docs/agents/sssc-planning/agent-overview.md

• The Related Files row for Instructions (| Instructions | .github/instructions/security/ |) remains technically correct as a directory reference, but the surrounding prose and diagram are inaccurate.

• docs/agents/sssc-planning/phase-reference.md and other files in the same directory do not reference the removed instruction file names and do not need updates.

• Reviewed and validated by a qualified human reviewer

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