feat(cli): implement cougr doctor toolchain diagnostics (#247)

[victor-134]

Summary

Implements `cougr doctor` — a first-class toolchain diagnostic command that verifies the local development environment before a developer hits a confusing build failure. This is part of the CLI epic (#238) and directly addresses the "works on my machine" class of onboarding failure.

Background

A working Cougr project needs the correct Rust toolchain, the `wasm32v1-none` compilation target, and a reasonably current `stellar` CLI installed. None of this is currently verified up front; a developer discovers a missing target or an outdated stellar CLI only when a build or deploy fails, often with an unhelpful error far from the actual cause.

This PR exposes that verification as a first-class `cougr doctor` command, consistent with the product strategy in `docs/strategy/06-product-strategy.md`.

Checks implemented (4)

#	Check	How it works	Fix command on failure
1	Rust toolchain	Runs `rustc --version`, parses version, compares against `rust-version = "1.70.0"` from root `Cargo.toml`	`rustup update stable`
2	wasm32v1-none target	Runs `rustup target list --installed` and checks for the Soroban WASM target	`rustup target add wasm32v1-none`
3	stellar CLI	Tries `stellar --version` then falls back to `stellar version`. Parses and compares against minimum 21.0.0	Link to Stellar CLI install docs
4	cargo	Sanity check with `cargo --version`	Rust install instructions

Each check produces a pass/fail result with an actionable fix command on failure — not just "missing" but the exact `rustup` / install command to fix it. A summary line reports `N/M checks passed` and exits non-zero if any check fails.

Design decisions

• Native Rust (no shelling out to external scripts) — consistent with the existing `check` and `verify` commands. Self-contained binary with no external dependencies beyond the tools being checked themselves.
• Simple semver comparison (`version_cmp`) — avoids adding a `semver` crate dependency. Handles the major.minor.patch comparison needed for both `rustc` and `stellar` version checks.
• Graceful degradation — if `stellar --version` can't parse the version string but the binary runs successfully, the check passes with a note rather than failing on a version-format edge case. This prevents false negatives when stellar CLI changes its output format.
• Unit tests — `parse_rustc_version`, `parse_stellar_version`, and `version_cmp` all have tests covering valid/invalid inputs and comparison edge cases.

Usage

```bash

Diagnose your toolchain

cougr doctor

Example output (all passing):

=== Cougr Doctor — Toolchain Diagnostics ===

✓ Rust toolchain

rustc 1.70.0 (≥ 1.70.0)

✓ wasm32v1-none target

wasm32v1-none target installed

✓ stellar CLI

stellar 21.0.0 (≥ 21.0.0)

✓ cargo

cargo 1.70.0

=== 4/4 checks passed ===

```

Files changed

File	Change	Lines
`cli/src/doctor.rs`	New — 4 toolchain checks, version parsing helpers, unit tests	+305
`cli/src/main.rs`	Modified — added `mod doctor`, `Doctor` subcommand variant, dispatch arm	+6

Definition of done (from #247)

• `cougr doctor` on a fully correct environment reports all checks passing with exit code 0
• `cougr doctor` with a missing `wasm32v1-none` target reports the specific fix command and exits non-zero
• `cougr doctor` with no stellar CLI on PATH reports a working install link/command rather than a generic "not found"
• Each failure prints the exact command to fix it
• Summary line: N/M checks passed with non-zero exit code if any check failed

Related

• Closes feat(cli): implement 'cougr doctor' toolchain diagnostics #247
• Part of epic Epic: ship the cougr CLI (new / add / check / doctor) #238
• Follows the same patterns as `cougr check` (feat(cli): implement 'cougr check' hygiene command #246) and `cougr check --verified` (feat(showcase): implement the 'Cougr Verified' badge check #258)
• Product spec: `docs/strategy/06-product-strategy.md`
• Roadmap: Phase 1 in `docs/strategy/13-roadmap.md`

[salazarsebas]

The core diagnostics (rustc, wasm32v1-none, stellar CLI, cargo) with actionable fix messages and the N/M summary look right. Two gaps against #247's DoD:

• MIN_RUST_VERSION is hardcoded ("1.70.0") instead of being read from the root Cargo.toml's rust-version field, which the issue calls out explicitly as the source of truth — this will drift silently if the minimum changes.
• Scope item 4 (cougr new invoking doctor automatically) isn't implemented here.

Also flagging: #272 already implements the same issue (#247) with the dynamic rust-version lookup, the cougr new --no-doctor integration, and broader test coverage via a MockRunner. To avoid two competing implementations landing, could you compare against #272 and either close this in favor of it, or pull in whatever this PR does better? Also note both PRs currently scaffold cli//cougr-cli independently — see the merge-order note on #288 for how we're sequencing that.

[salazarsebas]

Same base-drift issue as the other CLI PRs: this predates the cli/ scaffold on main, so merging as-is would drop cougr new, and it's currently showing as conflicting with main. Could you rebase and add doctor as an additional subcommand? Also flagging: #272 targets the same issue (#247) with a different structure (internal/cougr-cli/) — worth syncing with that author so we don't land two doctor implementations.