refactor: deduplicate logger mutex and withLock via lockable embedding

[lpcox]

Summary

Extracts the repeated mu sync.Mutex + withLock pattern into a shared lockable struct in global_helpers.go, then embeds it in all four logger types:

• FileLogger — removed standalone withLock method
• JSONLLogger — removed standalone withLock method
• MarkdownLogger — removed standalone withLock method
• ToolsLogger — removed standalone withLock method

Also adds CloseAllLoggers() convenience function that closes every global logger in a single call.

ServerFileLogger is intentionally excluded — it uses sync.RWMutex (read-write lock) rather than sync.Mutex.

Changes

File	Change
global_helpers.go	Added lockable struct + CloseAllLoggers()
file_logger.go	Embed lockable, remove withLock
jsonl_logger.go	Embed lockable, remove withLock
markdown_logger.go	Embed lockable, remove withLock
tools_logger.go	Embed lockable, remove withLock

Sub-issue resolution

• [duplicate-code] Duplicate Code Pattern: withLock delegation method across 4 logger types #3686 / [duplicate-code] Duplicate Code Pattern: Identical withLock methods in logger structs #3736 (withLock dedup): Fixed by this PR
• [duplicate-code] Duplicate Code Pattern: Log level dispatch quartet repeated across 3 logger façades #3687 (level dispatch quartet): Already addressed by existing logFuncs map in global_helpers.go
• [duplicate-code] Duplicate Code Pattern: CloseXxxLogger global function across 5 logger files #3688 (CloseXxxLogger wrappers): 1-line wrappers around generic closeGlobalLogger — minimal dedup value
• [duplicate-code] Duplicate Code Pattern: Duplicated write-and-sync logic in FileLogger and ServerFileLogger #3738 (write-and-sync): Different locking strategies (Mutex vs RWMutex) make dedup impractical

Fixes #3685
Fixes #3735

[Copilot]

Pull request overview

This PR refactors the internal/logger package to reduce repeated mutex boilerplate by introducing a shared embedded lockable type, and adds a single convenience entry point to close all global loggers.

Changes:

• Added a lockable embedded helper (mu sync.Mutex + withLock) to deduplicate per-logger locking wrappers.
• Updated FileLogger, JSONLLogger, MarkdownLogger, and ToolsLogger to embed lockable and removed their duplicated withLock methods.
• Added CloseAllLoggers() to close all global logger instances in one call.

Show a summary per file

File	Description
internal/logger/global_helpers.go	Introduces lockable and adds CloseAllLoggers() convenience closer.
internal/logger/file_logger.go	Embeds lockable and removes the local withLock wrapper.
internal/logger/jsonl_logger.go	Embeds lockable and removes the local withLock wrapper.
internal/logger/markdown_logger.go	Embeds lockable and removes the local withLock wrapper.
internal/logger/tools_logger.go	Embeds lockable and removes the local withLock wrapper.

Copilot's findings

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• Files reviewed: 5/5 changed files
• Comments generated: 2

[Copilot]

CloseAllLoggers introduces new exported behavior (ordering, first-error semantics, and “attempt all closes”). Please add unit tests to lock in: (1) all CloseXxx functions are invoked even if an earlier one returns an error, and (2) the first error is returned. Also consider updating the file-level comment that says helpers here “should not be called directly by external code”, since this function is intended as a public entry point.