feat(platform): wire target-aware PTY sessions

[decode2]

Linked issue

Closes #44

• The linked issue has the status:approved label.

Pull request type

Select exactly one option and apply the matching type:* label.

• Bug fix (type:bug)
• New feature (type:feature)
• Documentation (type:docs)
• Refactor (type:refactor)
• Maintenance or tooling (type:chore)
• Breaking change (type:breaking-change)

Summary

• Routes desktop PTY creation through target-aware platform services for shell, argv, PATH, environment, and Linux spawn authority.
• Preserves Windows session behavior and limits the xterm Windows PTY option to Windows targets.
• Adds focused backend authority and frontend session wiring coverage without changing lifecycle, backpressure, or restart behavior.

Changes

Area or file	Change
apps/desktop/src-tauri/src/lib.rs	Applies platform defaults and target authority when creating PTY sessions.
apps/desktop/src-tauri/src/platform/	Exposes target-aware shell and environment inputs while preserving Windows behavior.
apps/desktop/src-tauri/capabilities/linux.json	Grants the Linux PTY spawn authority required by the wired session path.
apps/desktop/src/ui/usePtySession.ts	Requests platform information and configures frontend sessions by target.
Rust and frontend tests	Covers authority manifests, platform wiring, PTY client calls, and Windows-only terminal options.
crates/splice-pty/src/lib.rs	Keeps backend session creation aligned with target-provided options.

Test plan

• Ran the focused automated checks: Rust platform 2/2, authority 3/3, resolver 2/2; frontend 54/54; typecheck, lint, fmt, cargo check, Windows cross-check, and candidate-scoped Clippy passed.
• Exercised the affected terminal, AI CLI, or desktop workflow: live Unix PTY harness passed 14/14 scenarios.
• Verified relevant Windows, Linux, or WSL behavior, or documented why it is not applicable: Linux compile-time and live PTY evidence passed; Windows cross-check passed. Native Windows and WSL runtime receipts remain task 3.3.

Verification warnings

• SDD verification passed with warnings and no blockers. Strict workspace Clippy is blocked only by two unchanged splice-clipboard dead-code warnings; candidate-scoped Clippy with -A dead_code passes.
• Linux/WSLg root-shell active-process detection does not yet identify child Codex/Claude processes. This behavior-activated limitation is intentionally deferred to task 3.3.

Rollback

Revert commit cfa2254 to remove this complete 10-path wiring unit. The prior platform authority boundary remains intact, and no unrelated or later task behavior is included.

Chained pull request context

• Previous PR: feat(platform): enforce Tauri command authority #43 (platform authority manifests v2)
• Next PR: task 3.3, runtime target/process validation
• Review this PR first: target defaults flowing through Tauri PTY creation, Linux authority, and Windows behavior preservation.
• Intentionally out of scope: native Windows/WSL runtime receipts, child AI CLI process targeting, and all task 3.3+ implementation.

PR6 #43 platform authority manifests
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PR7/16 [THIS] target-aware PTY session wiring
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PR8/16 task 3.3 runtime target/process validation (next)

Review budget: 388 authored lines across 10 paths (289 additions, 99 deletions).

Contributor checklist

• I searched for duplicate issues and pull requests before starting.
• I linked an approved issue using Closes, Fixes, or Resolves.
• I selected exactly one pull request type and applied exactly one matching type:* label.
• I completed every relevant template field and included reproducible test evidence.
• I updated documentation when behavior or compatibility changed, or confirmed no documentation change is required for this internal wiring slice.
• My commits follow Conventional Commits.
• My commits contain no AI attribution, including Co-Authored-By trailers.