fix(plan): extend the command's env context with prefix envs before interpreting it

[wan9chi]

Motivation

Command-prefix env assignments should shape planning the same way they shape the spawned shell command. Without that, a task like PATH=./bin tool can be planned against the wrong PATH, and nested task expansion can observe an env context that does not match the command the user wrote. This PR makes command-local env available at the point where the command is interpreted, so planning and execution agree.

Review focus

• The behavior change is intentionally narrow: command-prefix envs are added to the per-command planning context.
• Prefix env handling for spawn envs and cache fingerprints is otherwise unchanged.
• The snapshot coverage is meant to prove the user-visible failure mode: prefix PATH resolving a tool during planning.

Verification

• cargo check --all-features
• cargo clippy --all-features -- -D warnings
• cargo test
• cargo test -p vite_task_plan --test plan_snapshots

Draft while the stack is under review.

[wan9chi]

• fix(plan): extend the command's env context with prefix envs before interpreting it #440 👈 (View in Graphite)
• main

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Handle PATH prefix overrides case-insensitively on Windows

On Windows, environment variables are case-insensitive and the inherited path is commonly named Path; this insertion adds a separate PATH key instead of replacing the existing one. get_path_env later does a case-insensitive FxHashMap iteration, so with both Path and PATH present it can pick the inherited value and ignore a prefix like PATH=./bin tool, meaning executable lookup is still planned against the wrong path on Windows. This should mirror prepend_path_env and update the existing path key case-insensitively.

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