Document {id} placeholder in RECALL_*_CMD env vars

[tmad4000]

Issue

The RECALL_CLAUDE_CMD / RECALL_CODEX_CMD / RECALL_FACTORY_CMD / RECALL_OPENCODE_CMD env vars accept a {id} placeholder that recall substitutes with the session ID before launching. This is implemented in src/session.rs L102:

/// Env var format: "program arg1 arg2 {id}" where {id} is replaced with session ID
let cmd = cmd.replace("{id}", &self.id);

…but neither the README nor recall --help mentions the placeholder. As a result, a user who wants to add flags to the launch command (e.g. --dangerously-skip-permissions) will naturally write:

export RECALL_CLAUDE_CMD="claude --dangerously-skip-permissions"

…which silently drops the session ID. Selecting a Claude session in the TUI then launches a brand-new Claude (no --resume), and because Claude's stdin isn't a TTY in recall's exec context, it falls into --print mode and errors:

Error: Input must be provided either through stdin or as a prompt argument when using --print

recall list JSON also reflects the lossy resume command in the resume_command field, which makes the bug only visible if you happen to compare against the unset-env-var baseline.

Suggested fix

A one-line README addition under the customization / env-vars section, e.g.:

Note: Custom commands must include {id} where the session ID should go.
Example: RECALL_CLAUDE_CMD="claude --resume {id} --dangerously-skip-permissions"

Optionally, recall could:

1. Warn at startup if a RECALL_*_CMD is set without {id}.
2. Auto-append --resume {id} / resume {id} if the placeholder is missing and the command matches a known launcher.

Happy to send a PR for the README change if useful.

Repro

export RECALL_CLAUDE_CMD="claude --dangerously-skip-permissions"
recall list | jq '.sessions[] | select(.source=="claude") | .resume_command' | head -1
# → "claude --dangerously-skip-permissions"   (session ID dropped)

export RECALL_CLAUDE_CMD="claude --resume {id} --dangerously-skip-permissions"
recall list | jq '.sessions[] | select(.source=="claude") | .resume_command' | head -1
# → "claude --resume <uuid> --dangerously-skip-permissions"   (correct)

Version

recall 0.5.0 on macOS 15.4 (M5 MacBook).