Harness config 1.0.0-alpha.5

1.0.0-alpha.5 separates mutable-file ownership from ignore rules, updates
the CLI and conformance tests for that contract, and refreshes the public skill
and website guidance for safer full-repository adoption.

Mutable Files

• Adds .harnessMutable as the v1 declaration file for seed-once,
  runtime-owned files.
• Keeps .harnessIgnore focused on projection exclusion and target-output
  filtering; legacy [mutable] sections in .harnessIgnore now produce a
  validation diagnostic.
• Supports root, source-local, profile-local, and target-derived override
  .harnessMutable rules with gitignore-style last-match-wins behavior.
• Keeps target-output .harnessMutable files out of v1 so target-local
  filtering remains a .harnessIgnore responsibility.

Tooling And Tests

• Updates harnessc init, validation, planning, activation, and explanation
  flows to read .harnessMutable separately from .harnessIgnore.
• Preserves mutable files as create-once target state unless
  --force-mutable is explicit.
• Adds regression coverage for .harnessMutable parsing, legacy
  .harnessIgnore diagnostics, profile overlays, declaration-file
  non-projection, and dir/resource activation behavior.

Adoption Guidance

• Updates the Harness config skill to version
  2026-05-28.harness-mutable-contract with a plan-first migration workflow,
  skill-version reporting, a full-transition checklist, and a best-practice
  review checklist.
• Clarifies that mutable files must be copied into .harness as seed files
  before they are declared in .harnessMutable.
• Clarifies that composable root files such as AGENTS.md should be used only
  when composition, references, profiles, or overlays are needed; otherwise
  copy the root file directly.
• Updates website and LLM prompt guidance to recommend installing the public
  Harness config skill and following the migration checklist before editing.