feat(logger): add PICOCLAW_LOG_FILE env var for file-only logging

[liuy]

Summary

Add file-only logging support for picoclaw agent via PICOCLAW_LOG_FILE environment variable.

Motivation

Currently picoclaw agent outputs all logs to stdout/console, which can interfere with interactive use cases (e.g., TUI scenarios). Users need a way to redirect logs to a file only.

Usage

PICOCLAW_LOG_FILE=/tmp/agent.log picoclaw agent
PICOCLAW_LOG_FILE=~/.picoclaw/logs/agent.log picoclaw agent

[yinwm]

Related PR: #1853

This PR has a dependency relationship with #1853 (feat(logging): add configurability for log levels preference).

Conflict Analysis

Both PRs modify cmd/picoclaw/internal/agent/helpers.go:

This PR (#1828)	#1853
Adds logger.ConfigureFromEnv() at function start	Moves debug flag handling after LoadConfig()
Modifies pkg/logger/logger.go (adds DisableConsole, ConfigureFromEnv)	Modifies pkg/logger/logger.go (adds ParseLevel, SetLevelFromString)

Recommended Merge Order

#1853 should be merged first, then this PR needs to be rebased.

Reason:

• feat(logging): add configurability for log levels preference #1853 changes the initialization order (moves LoadConfig() before debug flag check)
• This PR adds ConfigureFromEnv() which should be called after SetLevelFromString() from feat(logging): add configurability for log levels preference #1853
• Merging feat(logging): add configurability for log levels preference #1853 first provides a stable base for this PR to rebase onto

Suggested Integration (after #1853 is merged)

```go
func agentCmd(...) error {
// 1. Load config first (sets log level via SetLevelFromString internally)
cfg, err := internal.LoadConfig()

// 2. Apply env var overrides (file logging can override config)
logger.ConfigureFromEnv()  // ← This PR

// 3. Debug flag has highest priority
if debug {
    logger.SetLevel(logger.DEBUG)
}
...

}
```

Feature Summary

• feat(logging): add configurability for log levels preference #1853: Controls what log level to use (config + env)
• This PR: Controls where logs go (file vs console)

These two features are complementary and work together:
```bash
PICOCLAW_LOG_FILE=/tmp/agent.log picoclaw agent # This PR: logs go to file

config.json: {"log_level": "debug"} # #1853: logs at debug level

```

[yinwm]

@liuy plz resolve conflicts.

[liuy]

@liuy plz resolve conflicts.

Done as commented

[yinwm]

LGTM! Clean implementation with good test coverage. Approved.