fix(skill): preserve incident comment content

[ysyneu]

Root cause

Audit run audit-20260722T031714Z-auto found session sess_nnZqgRwBRbo2u7YpCpLDSS constructing an incident comment inside double-quoted Bash. Markdown backticks were evaluated as command substitutions before fduty received the argument, so the API stored altered content. The workflow then reported success without reading the mutation back.

End-state fix

• Construct multiline comments with a quoted heredoc whose fresh delimiter is absent from the intended content.
• Require timeline read-back after every comment write before success is reported.
• Cover literal Markdown backticks and a collision-shaped legacy delimiter line with a deterministic shell regression.

Verification

• make GOLANGCI_LINT=/Users/ysy/go/bin/golangci-lint
• go test -race -count=1 ./...
• go run ./internal/cmd/skilldoc check
• Full branch and post-PR adversarial reviews: ready, no remaining findings

The repository-cached linter was built with Go 1.24 and cannot analyze this Go 1.25.1 module; the same make target passes with the Go 1.25-built linter shown above.