install: offer to replace existing shell integration (interactive) + --force

Problem

tryme install refuses when the # try shell integration marker already exists in the rc file:

try is already installed in /Users/x/.zshrc
To reinstall, remove the '# try shell integration' block first.

The installed block embeds the absolute path of the binary that ran install (resolve_self_path), so this bites on every channel switch — brew ↔ cargo ↔ dev build — and the remedy is hand-editing the rc file, which is exactly the surgery install exists to spare people. The eval "$(tryme init …)" form sidesteps it, but install should be able to maintain what it wrote.

This is upstream-inherited behavior (try.rb marker check), so shipping it is a deliberate divergence to be recorded per ADR-0003 — optionally proposed upstream first if we want the behaviors to converge, owner's call.

Proposal

Two additive changes, no change to any existing exit code or script-observable behavior (conformance suite unaffected — install has no upstream conformance test):

1. tryme install --force — replace the marker-delimited block instead of refusing: strip from the # try shell integration line through the wrapper function it wrote, append the freshly generated block. Non-interactive-safe; second --force run is a no-op rewrite.

2. Interactive default: prompt instead of refuse. Marker found + tty session → ask on stderr:

   try is already installed in ~/.zshrc — replace it? [y/N]
   

   N/default preserves today's behavior exactly (message + exit 0). Non-tty keeps the current refuse-with-hint unconditionally, so scripts/CI never block on a prompt. Read the answer from /dev/tty, consistent with the selector's stream discipline.

Safety

• Replacement bounded strictly by the block install writes (marker + emitted function) — never pattern-match user-authored try functions.
• Read-only rc files keep the existing warning + exit 1 path.
• New --force flag joins the clap declaration + try.usage.kdl via the inventory-diff chain; hidden nothing.

Sequencing

Post-1.0 surface per ADR-0004 staging, or earlier if the owner rules the divergence in — it needs an ADR-0003 ruling-log entry either way.

Start at the install command's marker check and resolve_self_path, then trace the clap declaration and try.usage.kdl inventory-diff chain. Read docs/adr/0003-divergence-authority.md and the ADR-0004 staging reference before deciding scope. Done means --force replaces only the generated block, interactive TTY sessions prompt, non-TTY sessions preserve refusal behavior, and existing exit codes and read-only handling remain unchanged.