Unify acquired graph reads and operation-scoped configuration

[hlubek]

External adapters need graph content, configuration and attachments to come from the same selected revision throughout a read operation. Unify acquisition across application reads, add branch-aware attachments, and reuse workflow dependency configuration per operation while checking authorization independently.

Preserves existing local configuration precedence and write validation/retry behavior. Includes adapter adoption guidance and regressions for concurrent branch changes and source cleanup.

Validation: full tests including the nested example, focused race tests, vet, lint and build passed.

Greptile Summary

This PR consolidates graph, configuration, dependency, search, workflow, and attachment reads around operation-scoped acquired snapshots.

• Adds branch-aware read acquisition and forwards session branch selection through MCP.
• Applies acquired configuration to operation-local runtime clones while preserving authorization checks.
• Adds source cleanup handling and regression coverage for branch advancement and acquisition failures.
• Moves local attachment path ownership to the selected source, though the current retained-snapshot path can diverge from its immutable content.

Confidence Score: 4/5

The PR should not merge until local attachment paths are guaranteed to identify the same revision-fixed bytes returned by the acquired source.

Retained attachment reads serve content from an immutable in-memory snapshot but return a path into the mutable live checkout; once an apply advances that checkout, local clients can read a different revision through the advertised path.

Files Needing Attention: pkg/local/read_snapshot.go

Important Files Changed

Filename	Overview
pkg/application/read_snapshot.go	Introduces unified snapshot selection, operation-local acquired configuration, legacy-store compatibility, and centralized release handling.
pkg/application/application.go	Migrates application read APIs to acquired snapshots and branch-aware attachment and information reads.
pkg/application/workflow.go	Reuses materialized graph views and effective configuration within each workflow operation.
pkg/local/read_snapshot.go	Adds source-provided attachment paths, but derives them from a mutable checkout while serving immutable snapshot bytes.
pkg/mcpapp/tools.go	Forwards session branches into information and attachment reads and exposes source-provided paths to local clients.
cmd/sdd/serve.go	Composes branch-aware local snapshot acquisition without using mutation acquisition.

Prompt To Fix All With AI

### Issue 1
pkg/local/read_snapshot.go:36
**Local Path Can Drift**

The attachment content is read from the immutable snapshot filesystem, but `LocalPath` is built from the mutable live checkout directory. If a concurrent or later apply replaces that file before the local MCP client follows the path, the client reads bytes from a different revision than the content returned by this operation. Only return a path that remains tied to the acquired snapshot, or omit it when that cannot be guaranteed.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (2): Last reviewed commit: "sdd: signal tactical Should local MCP at..." | Re-trigger Greptile}

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