Fix ToSlogHandler duplicating time and level in output

[umputun]

Fix lgrSlogHandler.Handle() prepending its own timestamp and level to the message before passing to lgr.Logf, which adds its own formatting — resulting in doubled time and level in output. Closes #29

• Remove redundant time construction from Handle(), keep level prefix for lgr's extractLevel detection
• Add TestSlogHandlerNoDuplication verifying INFO/DEBUG levels and timestamp appear exactly once
• Fix existing test assertions to match correct lgr output format (padded levels)
• Fix Go version in CLAUDE.md (1.20 to 1.21 per go.mod)

[Copilot]

Pull request overview

Fixes ToSlogHandler output duplication by preventing lgrSlogHandler.Handle() from injecting its own timestamp/level formatting before delegating to lgr.Logf, and updates/extends tests to validate the corrected output format.

Changes:

• Remove redundant timestamp construction from lgrSlogHandler.Handle() to avoid double time/level in output.
• Update slog integration tests for padded level formatting and add a new no-duplication test.
• Add CLAUDE.md development guidelines (notably Go 1.21).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
slog.go	Removes timestamp prefixing in Handle() and adjusts how the message is composed before calling Logf.
slog_test.go	Updates assertions for padded INFO/WARN, adds TestSlogHandlerNoDuplication, and adjusts TRACE-level test setup/record creation.
CLAUDE.md	Adds repository development guidelines reflecting Go 1.21 and local conventions.

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[Copilot]

Handle passes the fully formatted logMsg as the format string to h.lgr.Logf(logMsg). For lgr.L implementations that delegate to fmt.Printf/log.Printf (e.g. lgr.Std), any % in the slog message or attribute values will be interpreted as formatting verbs, corrupting output (and can emit %! artifacts). Prefer calling Logf with a constant format (e.g., h.lgr.Logf("%s", logMsg)) so messages are treated as data, not a format string.

Suggested change

	h.lgr.Logf(logMsg)
	h.lgr.Logf("%s", logMsg)

[Copilot]

The duplication assertions use substring counting / NotContains on the entire log line (e.g., strings.Count(line, "INFO")). This can produce false failures if the message or attribute values legitimately contain the same token (e.g., value contains "INFO" or "DEBUG"). Consider asserting on the structured prefix instead (regex anchored to the start, or stripping the initial timestamp+level and then checking the remainder doesn’t start with another timestamp/level).

Suggested change

	// must not contain INFO for a debug message
	assert.NotContains(t, line, "INFO")
	// ensure the structured prefix has DEBUG as the level (and not INFO)
	levelRe := regexp.MustCompile(`^\d{4}/\d{2}/\d{2}.*\bDEBUG\b`)
	assert.True(t, levelRe.MatchString(line), "expected DEBUG level in structured prefix, got: %s", line)