Docs: consolidate workflow/policy content duplicated across 4-6 files

[codeforester]

Goal

Consolidate the issue-branch-worktree workflow, "Pre-Edit Workflow Gate," and
superseded-PR closure procedure to a single canonical document, with other
files linking to it instead of restating it.

Background

A documentation review of the process/CI/agent docs found the same workflow
content independently restated across many files, which is a real drift risk
(one copy has already partially drifted — see the companion GitHub Project
Initiative-duplicate issue for a concrete example of restated content going
stale):

• The "Pre-Edit Workflow Gate" paragraph is restated almost verbatim in three
  files: AGENTS.md:25-39, docs/github-workflow.md:7-20, and
  .ai-context/WORKFLOWS.md:3-16.
• The superseded-PR closure procedure is fully restated four separate times:
  AGENTS.md:85-98, skills.md:40-56, docs/github-workflow.md:461-508,
  and .ai-context/WORKFLOWS.md:134-139.
• The full issue/branch/worktree workflow (labels, branch shape, Project
  status transitions) is independently restated across six files:
  CONTRIBUTING.md:14-71, AGENTS.md:23-101, skills.md:6-38,
  STANDARDS.md:661-691, all of docs/github-workflow.md, and
  .ai-context/WORKFLOWS.md:18-104. Only docs/github-workflow.md needs the
  full detail; the rest should point to it.
• skills.md:16 narrows the auto-assign rule to "Codex-created issues" while
  every other file (AGENTS.md:47-49, STANDARDS.md:668-669,
  docs/github-workflow.md:79-80, .ai-context/WORKFLOWS.md:31) uses
  tool-neutral "agent-created"/"automation" language — an example of the
  restated copies already diverging in wording.

Scope

• AGENTS.md
• CONTRIBUTING.md
• skills.md
• STANDARDS.md
• docs/github-workflow.md
• .ai-context/WORKFLOWS.md

Acceptance Criteria

• docs/github-workflow.md is the single canonical source for the full
  issue/branch/worktree workflow, Pre-Edit Workflow Gate, and
  superseded-PR closure procedure.
• AGENTS.md, CONTRIBUTING.md, skills.md, STANDARDS.md, and
  .ai-context/WORKFLOWS.md each keep only the minimum summary needed
  for their own audience and link to docs/github-workflow.md for full
  detail, instead of restating it.
• skills.md's "Codex-created" wording is aligned with the tool-neutral
  "agent-created" language used elsewhere.

Validation

git diff --check; manual read-through confirming each file still makes
sense standalone with the trimmed summary + link.

Non-Goals

• Changing the actual workflow policy — this is a consolidation/DRY issue,
  not a policy change.

Project Metadata

• Priority: P3
• Size: L
• Area: Docs
• Initiative: Contract Hardening
• Agent Assignment: Human first (requires editorial judgment on what to keep
  as a local summary vs. trim entirely in each of six files)