fix: correct MSRV to 1.86, repair mojibake, drop unusable crates.io publishing

[marcuspat]

Follow-up to #7, which merged while these were in flight. Three items, one of them a build break the UAT report scored as PASS.

docker build cannot succeed today

The builder image is rust:1.82-slim, but the dependency graph has moved past it:

Dependency	Declared rust-version
ratatui 0.30	1.86.0
clap 4.6	1.85

Cargo refuses the workspace before compiling anything. Same class as the Go-version mismatch flagged in spacelift-intent as SBLD-007 — the report scored Sentinel's build/deploy section 7/7.

• Builder → rust:1.86-slim
• rust-version = "1.86" added to workspace.package, so cargo reports the real floor instead of failing deep inside a dependency
• README's "Requires Rust 1.75+" and CONTRIBUTING's "1.75 or newer" corrected

The TUI printed mojibake

Box-drawing rules, em dashes, arrows and ellipses were committed as double-encoded UTF-8 — the byte sequence you get by decoding real UTF-8 as Latin-1 and re-encoding. These are output strings, not just comments:

println!("── Investigating ──");   // rendered: â──â── Investigating â──â──
"Investigation complete — {} observation(s) collected."   // rendered: complete â€" 3 observation(s)

3,096 sequences repaired across the four sentinel-tui sources and the Dockerfile. The repair re-encodes each run to Latin-1 and decodes it as UTF-8, so every character is restored to what was originally typed rather than guessed.

crates.io publishing was never going to work

Crate	Status on crates.io
sentinel-agent (the README badge)	does not exist, never published
sentinel-core	registered to Forsworns since 2022
sentinel-tui	registered to mateusdcc

The publish job added in #7 could not succeed under these names. Per your call: badge and publish job removed. Releases ship binaries and container images; the four-target build with SHA-256 sums and CHANGELOG-derived release notes is untouched. Nothing forecloses publishing later under an available name.

Verification

cargo clippy --workspace --all-targets -- -D warnings clean and cargo test --workspace 413 passing against this tree. Every file was verified by git blob SHA against the locally built and tested copy.

Still open

cargo fmt --all -- --check fails across ~250 sites on current stable. CI doesn't run it and I left it out of the release workflow rather than bundle a whole-tree reformat here — cargo fmt --all on this branch is a one-command follow-up if you want it, and cargo fmt --check can go into CI once the tree is clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WhYNu2HEX62gqqtcPM5twr