[fix] Require config for MCP cleanup tools

[Jia-Ethan]

Summary

• Require initialized rimba config before MCP clean, remove, and status handlers do any work.
• Reuse the loaded config's default source for clean/status main-branch resolution.
• Add regression coverage for the uninitialized-repo path on all three MCP tools.

Test Plan

• Unit tests pass (make test)
• Linter passes (make lint)
• E2E tests pass (make test-e2e)

Additional focused checks run:

GOPROXY=https://goproxy.cn,direct go test ./internal/mcp -run 'RequiresConfig' -count=1
GOPROXY=https://goproxy.cn,direct go test ./internal/mcp -count=1
GOPROXY=https://goproxy.cn,direct go test ./... -count=1

Risk and scope: scoped to MCP handler preflight checks and tests. Existing required-argument errors still run before the config gate, while valid tool calls now fail with the same friendly initialization message used by the other MCP tools when .rimba/settings.toml is unavailable.

Issue

Closes #267

[lugassawan]

High — prune mode now unnecessarily requires config

mcpCleanPrune calls only git.Prune and git.PruneRemotes — it never reads cfg.DefaultSource. By placing requireConfig() before the switch, all three modes (including prune) now fail without an initialized repo, extending the blast radius beyond the issue scope.

prune was previously usable in any repo; this is a silent behavioral regression.

Suggested fix: move the config gate inside the merged and stale case arms, where cfg.DefaultSource is actually consumed:

switch mode {
case "prune":
    return mcpCleanPrune(ctx, r, dryRun)
case "merged":
    cfg, cfgErr := hctx.requireConfig()
    if cfgErr != nil {
        return mcp.NewToolResultError(cfgErr.Error()), nil
    }
    return mcpCleanMerged(ctx, r, cfg.DefaultSource, dryRun, force)
case "stale":
    cfg, cfgErr := hctx.requireConfig()
    if cfgErr != nil {
        return mcp.NewToolResultError(cfgErr.Error()), nil
    }
    return mcpCleanStale(ctx, r, cfg.DefaultSource, dryRun, staleDays, force)

Then update TestCleanToolRequiresConfig to use modeMerged or modeStale (since modePrune would no longer require config).

[lugassawan]

Review Decision: COMMENT

Reviewed by reviewer (swe-workbench). Posted 1 inline comment, deduped 0.

Informational (out-of-diff)

internal/mcp/tool_list.go — handleListArchived bypasses the config gate (pre-existing)

handleList short-circuits to handleListArchived before calling requireConfig() when archived=true. handleListArchived uses the nil-safe configDefault(hctx) helper, so list --archived still works without an initialized repo. This PR does not introduce or worsen it — it predates this change — but it is the same class of gap issue #267 addressed on the other three tools. Worth a follow-up issue.