spec(SDD-004): scaffold session-start-config SSOT spec [AGENT-DRAFT tags pending review]

[mlorentedev]

Summary

• Scaffolds SDD-004-session-start-config spec folder (proposal + tasks + verification, 167 LOC of markdown). No source/test changes.
• Locks the contract for the future SDD-004 implementation PR (claude-session-start.{sh,ps1} → thin readers of session-start-config.json SSOT, mirroring doctor.{sh,ps1} + env-contract.json).
• AUDIT-002 ranks this as the Modernize dotfiles with Stow and testing #1 SSOT-extractable candidate in the repo (937 LOC combined, HIGH change frequency).

State

⚠ Proposal sections are currently tagged [AGENT-DRAFT — review before archive] (per /spec fill skip semantics — user asked agent to generate drafts). The archive lock is active; /spec archive SDD-004 will refuse until tags are converted to plain content. Reviewer: please skim §Why / §What / §Risks and either accept-as-is (remove the [AGENT-DRAFT] line) or edit before this PR merges.

Blockers documented in §Risks

• 🔴 R1 (BLOCKER, must resolve before code): byte-equivalence. SessionStart fires on every Claude session; first implementation task MUST be capturing golden output (one per OS) and asserting byte-identity post-refactor.
• 🟡 R2: PowerShell ASCII-only rule (pattern-powershell-ascii-only — hit twice already). JSON file must be ASCII-only; no em-dash / smart-quote in probe messages or comments.
• 🟡 R3 (open question, resolve before tasks.md freeze): schema shape — flat list of injectors [{id, probe, content_source, on_failure}, ...] (recommended) vs nested by category. AUDIT-002 explicitly warns "keep schemas minimal".

Why a spec-only PR

• Locks the contract publicly before implementation begins (~300 LOC P1 Med-risk).
• Satisfies the spec-gate CI guardrail (PR feat(sdd,agents): SDD-003 spec-gate + AI-019 model-tier policy #60) for the future implementation PR.
• Keeps the implementation PR diff focused on code, not also-paragraphs.

Test plan

• markdownlint not configured — N/A
• Spec folder contains all 3 templates per pattern-spec-driven-development
• Reviewer: skim §Why / §What / §Risks and either approve or edit [AGENT-DRAFT] content
• Reviewer: pick a schema shape for R3 (recommended: flat list)
• Once [AGENT-DRAFT] tags removed → merge → open feat/SDD-004-session-start-config implementation branch