docs: document Mustache template variables for custom rule messages

[kitsuyui]

Custom rules can use Mustache variables in their message field, but the README
only showed the variables implicitly in the sample code ({{commitId}}, {{matched}}).
Users had to read source code to discover all available variables or to understand that
kind: "pr" and kind: "commit" expose different variable sets.

Changes

• README.md — add a "Template variables" subsection under "Custom rules" with a
  reference table listing {{prNum}} (pr), {{commitId}} (commit), and {{matched}}
  (both) with their availability and description.

Verification

• typos README.md — no spelling issues
• README-only change; action logic is unaffected

[github-actions]

🎉 Happy commit!

• dogfooding: PR docs: document Mustache template variables for custom rule messages #841 always celebrates.
• dogfooding: e4480af169cf18009add9a90aa3fd1eafd0c2568 always celebrates.

[github-actions]

Code Metrics Report

	main (6709cec)	#841 (f7e1de4)	+/-
Coverage	99.2%	99.2%	0.0%
Code to Test Ratio	1:0.5	1:0.5	0.0
Test Execution Time	1s	1s	0s

Details

  |                     | main (6709cec) | #841 (f7e1de4) | +/-  |
  |---------------------|----------------|----------------|------|
  | Coverage            |          99.2% |          99.2% | 0.0% |
  |   Files             |              5 |              5 |    0 |
  |   Lines             |            134 |            134 |    0 |
  |   Covered           |            133 |            133 |    0 |
  | Code to Test Ratio  |          1:0.5 |          1:0.5 |  0.0 |
  |   Code              |           1678 |           1678 |    0 |
  |   Test              |            955 |            955 |    0 |
  | Test Execution Time |             1s |             1s |   0s |

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Reported by octocov