Extract process support into a focused SwiftPM module and test target

[morluto]

Why

swift test --filter narrows execution, not target compilation. Small process-lifecycle tests still compile the broad RepoPromptApp and root test modules.

This PR tests the architectural remedy on one bounded area. It extracts process execution and lifecycle support without attempting a repository-wide target rewrite.

Advances #416.

What changes

• Adds the internal RepoPromptProcessSupport SwiftPM target.
• Moves the process runner, launcher, termination, environment, framing, and required concurrency primitives into it.
• Makes RepoPromptApp depend on the lower-level module.
• Adds RepoPromptProcessTests and moves the lifecycle canary out of the root test target.
• Uses package access instead of widening the module to public API.
• Updates imports, the curated test ledger, formatting paths, source-layout guardrails, Sentry in-app classification, and Xcode workspace generation.

Boundary

RepoPromptApp
    ↓
RepoPromptProcessSupport
    ↓
RepoPromptShared

Existing features remain consumers. No compatibility wrapper or duplicate implementation is introduced.

Deliberate scope

This extracts one coherent canary only. It does not split every infrastructure area, redesign process APIs, or change conductor/cache behavior.

Review order

1. Package.swift
2. Sources/RepoPromptProcessSupport
3. Tests/RepoPromptProcessTests
4. access/import changes
5. ledger and guardrails
6. Xcode generator contract

Most changed lines are file movement. Review the target graph and module boundary first.

Verification

• source-layout guardrails
• test-suite optimizer contract tests
• Xcode generator topology test
• SwiftFormat checks
• package manifest resolution
• contribution commit/push preflights

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[baron]

Two exact-head blockers remain at 098a5cd419095ba34491a17c761dd4839b2f3f45: (1) ProcessTerminationError is package-visible but its LocalizedError.errorDescription witness is only internal (ProcessTermination.swift:44,48-55), producing the hosted compiler failure; (2) test_xcode_workspace_generator.py:112-116 passes the corrected dependency set as assertEqual's message rather than the expected value, so the contract test still checks the incomplete set. Please fix both, resolve current-main conflicts, and rerun the build and Xcode-generator checks.

[baron]

Blocking on replacement head 3ac5445bce18f5b0192e10e61281ffcce50cddb1.

The previous access-witness and Xcode-generator assertion findings are fixed, but the new module split does not compile: RepoPromptProcessSupport exposes/uses ChildProcessExitObserver from ProcessTermination.swift, while that type remains owned by the app target under Sources/RepoPrompt/Infrastructure/Process/ChildProcessExitObserver.swift. The lower-level module cannot see the app-owned type, and all app shards plus the Sentry build fail.

Move the observer contract into the owning lower-level module (or remove the cross-module exposure), add the appropriate module-boundary coverage, and rerun all exact-head build/test lanes.