docs: establish native context folding S01 baseline

[LeXwDeX]

Why

Issue #605 requires the source and delivery contracts to be frozen before any dynamic context-folding implementation begins. The two session stacks differ in message shape, tool registration, provider transformation, compaction entry points, and test artifact paths; these seams need positive, testable mappings before a default-on implementation can start.

Closes #605

This draft records S01 only and must remain unmerged until S02–S10 complete; S01 itself does not close the issue.

What changed

• Added the approved native context-folding design, implementation plan, and an evidence-backed S01 audit.
• Fixed Core read acceptance to positively map verified ReadTool.TextPage.content while requiring complete input and outer-field equality and preserving the JSON result type, schema, and stored history.
• Fixed Claude/Mistral acceptance to reuse one extracted host call-ID normalizer, validate final transformed call/result pairs, and positively cover non-colliding mappings; only collisions or unknown transforms skip.
• Adopted the bounded session/instance source-identity ledger as the formal provenance design. It stores no input, output, path, fingerprint, candidate, or plan and is distinct from a forbidden cross-request dedup cache.
• Corrected the license wording to independent implementation from the project specification without direct external-source copying; the documents do not claim a legal clean-room process.
• Recorded the actual Issue/PR, Bun and SpecGit versions, hook state, document-formatting result, and delivery gates.
• Changed no product code and did not enter S02, enable the feature, merge, deploy, or publish.

Evidence

• Baseline: origin/dev@4b554abd9dac6ae93c46b555b60dcb478eff7be6.
• Initial S01 commit: 7bbc09761afe8ea4f22cc01d10ef6c799bf87708.
• Audit-correction commit: 22576d58a8411f5c59de2e4f0bd7a553565de517.
• Full audit: docs/context-folding/evidence/S01.md.
• The graph located ReadTool.TextPage at packages/core/src/tool/read-filesystem.ts:77-84; coverage reported no recorded issue and direct source reading confirmed type/content/mime/offset/truncated/next.
• Prettier 3.6.2 initially rejected the three edited Markdown files; after formatting, prettier --check passed and git diff --cached --check passed.
• Pinned Bun is 1.3.14; local Bun is 1.4.2. The first push did not run pre-push typecheck because core.hooksPath=.husky/_ but that wrapper directory is absent, so the documentation does not report a local typecheck pass.
• SpecGit created Issue feat: add native dynamic context folding #605 and Draft PR docs: establish native context folding S01 baseline #606. The request targets dev, remains draft, and the branch head is the audit-correction commit.
• This documentation-only step has no runtime artifact, so no product deployment applies. Product tests and dev prerelease acceptance remain S02–S10 work.

Checklist

• The diff is limited to the two approved specs and the S01 evidence record.
• A01–A05 corrections are represented in the formal design, step plan, test matrix, and evidence record.
• Every material claim is tied to current-source lines, graph coverage metadata, or raw command output.
• The branch is pushed for parent-task re-review and remains draft, unmerged, and undeployed.
• Parent task has approved S01 and authorized S02.