ci: add CLAUDE.md and Copilot custom instructions (#1830)

* ci: add CLAUDE.md and GitHub Copilot custom instructions

Add AI assistant instructions to improve automated review quality:
- CLAUDE.md: project conventions, RST rules, linter details
- .github/copilot-instructions.md: repo-wide Copilot context
- .github/instructions/rst-linter.instructions.md: linter rules
  for RST files (address space, line length, suppression syntax)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: fix build commands, code-block exemption, and address space heading

- Fix Docker build/run commands to match README (vyos/vyos-documentation
  image, /vyos mount, /vyos/docs workdir)
- Fix local build to run from docs/ directory
- Clarify code-block lines are exempt from 80-char limit
- Expand address space heading to reference docs/documentation.rst
- Fix confusing 0.0.0.0/0 entry in common mistakes
- Simplify inline code backtick example

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: expand Copilot instructions to reduce false positive reviews

Add guidance for common false positives observed in automated reviews:
- Only flag issues introduced by the PR, not pre-existing
- Column-0 linter markers are the repo convention, not a bug
- Code-block lines are exempt from 80-char limit
- RFC 1918 and other private IPs are allowed without suppression
- TODO markers can be intentionally added for tracking
- YAML in code blocks uses RST-relative indentation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: fix backtick rendering and linter marker indentation guidance

- CLAUDE.md: use fenced code span to show RST double-backtick syntax
- copilot-instructions.md: markers match surrounding indentation inside
  directives, column 0 at top level (both patterns exist in repo)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/.github/instructions/rst-linter.instructions.md b/.github/instructions/rst-linter.instructions.md
new file mode 100644
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+---
+applyTo: "**/*.rst"
+---
+
+## RST Linter Rules
+
+Every pull request is automatically linted by `doc-linter.py` (from the `vyos/.github` repository). It checks **only changed files** for two things: IP address usage and line length.
+
+### IP Address Rules
+
+The linter rejects public IP addresses that are not reserved for documentation.
+
+**Use these documentation-reserved addresses (no suppression needed):**
+
+| Type | Range | RFC |
+|------|-------|-----|
+| IPv4 | `192.0.2.0/24` (TEST-NET-1) | RFC 5737 |
+| IPv4 | `198.51.100.0/24` (TEST-NET-2) | RFC 5737 |
+| IPv4 | `203.0.113.0/24` (TEST-NET-3) | RFC 5737 |
+| IPv6 | `2001:db8::/32` | RFC 3849 |
+
+**These are allowed without suppression (not flagged by linter):**
+
+| Type | Range | Why |
+|------|-------|-----|
+| RFC 1918 private | `10.0.0.0/8` | Private address space |
+| RFC 1918 private | `172.16.0.0/12` | Private address space |
+| RFC 1918 private | `192.168.0.0/16` | Private address space |
+| Loopback | `127.0.0.0/8` | Loopback range |
+| Link-local | `169.254.0.0/16` | Link-local |
+
+**These require `stop/start_vyoslinter` suppression:**
+
+| Example | Why suppression is needed |
+|---------|--------------------------|
+| `8.8.8.8` (Google DNS) | Real public IP, not documentation-reserved |
+| `64:ff9b::/96` (NAT64) | Well-known prefix, not in doc ranges |
+| Real provider IPs in examples | Authenticity matters for the example |
+
+### Line Length Rules
+
+Maximum 80 characters per line. Lines inside `.. code-block::` directives are **exempt** from the line length limit (they render with `<pre>` tags and preserve source formatting).
+
+### Suppression Syntax
+
+When real public IPs or long lines are unavoidable, wrap the block:
+
+```rst
+.. stop_vyoslinter
+
+.. code-block:: none
+
+   set system name-server '8.8.8.8'
+
+.. start_vyoslinter
+```
+
+**Rules for suppression markers:**
+
+- Place `.. stop_vyoslinter` on its own line, immediately before the content
+- Place `.. start_vyoslinter` on its own line, immediately after the content
+- Always re-enable the linter — never leave it stopped for the rest of the file
+- Suppress the smallest possible region
+- Do NOT use suppression for content that can use documentation-reserved addresses instead
+
+### Common Mistakes
+
+- Removing `stop/start_vyoslinter` markers without fixing the underlying issue (exposes the line to the linter and fails CI)
+- Using real public IPs when documentation addresses would work just as well
+- Adding suppression markers around content that only has private (RFC 1918) addresses or `0.0.0.0/0` — these are allowed and don't need suppression