feat: cargo dev-install to symlink dev build into PATH

[delianides]

Summary

• Adds cargo dev-install: builds the debug perch and symlinks target/debug/perch into ~/.local/bin/perch, so every later cargo build is instantly live — no reinstall. cargo run -p xtask -- uninstall removes the symlink (only if it's a symlink; never deletes a real file).
• Implemented via a new xtask workspace crate (dev tooling only — publish = false, dist = false, never shipped). Converts the repo to a Cargo workspace; the root perch package and cargo-dist release flow are unchanged.
• Symlink logic has clobber protection (refuses to overwrite a real file) and is idempotent. The install command warns if ~/.local/bin isn't on PATH or if an earlier PATH entry shadows the dev build. Hardened against relative CARGO_TARGET_DIR and missing build output.
• Dev build version string: a new build.rs captures the git short SHA at compile time, so debug builds report perch 1.6.0-dev.<sha> (and …-dev.<sha>.dirty with uncommitted changes). Gated on cfg!(debug_assertions), so --release builds (incl. Homebrew/cargo-dist) stay clean 1.6.0. Makes it obvious at a glance you're running the dev build.
• Unix-only by design; manages perch only (not git-perch).
• Docs updated in README.md and CLAUDE.md.

Test Plan

• cargo test --workspace — passes (6 xtask helper tests: fresh link, idempotent relink, clobber refusal, unlink, missing-noop, real-file refusal; plus a VERSION-format test)
• cargo clippy --workspace clean
• Manual: cargo dev-install → symlink created, perch --version shows 1.6.0-dev.<sha>; second run idempotent; uninstall removes it; PATH shadow warning fires when another perch is earlier on PATH
• cargo build --release → perch --version reports clean 1.6.0 (release/Homebrew output unaffected)
• Confirm dist plan still ships only perch (xtask excluded) — optional, needs cargo-dist installed

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