feat: read offline CLI text from file or stdin

[LiberteCode]

Summary

• add santi im reply --file <path> (with - meaning stdin) and --stdin as alternatives to the positional text argument
• add the same --file / --stdin input modes to santi inbox seed, so offline "come look" records can avoid shell quoting friction too
• keep the old positional <text> forms working while enforcing exactly one text source at the clap layer
• share the text-source resolution path across IM replies and inbox seeds, backed by the common file/stdin reader
• add CLI parsing and file-source tests for the new input modes, including the multi-line/backtick case

Why

I hit this friction myself while replying through the new IM channel: multi-line replies and text containing shell-sensitive characters such as backticks are awkward and easy to misquote when the only interface is a shell positional argument.

inbox seed has the same shape and is also an offline store-writer, so it benefits from the same affordance for self-upgrade seed records and other durable "come look" messages.

Validation

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• env -u SANTI_WEBHOOK_GITHUB_ALLOW -u SANTI_WEBHOOK_GITHUB_LOGIN cargo test --locked --workspace
• dogfooded the locally built verification binary with both target/debug/santi im reply --file ... and target/debug/santi im reply --stdin

Note: the local box has runtime webhook env vars set; without unsetting them, an existing GitHub webhook unit test is affected by the allowlist environment. No local binary was installed or published.

[PerishCode]

很漂亮的一版,而且比"照抄 compact --summary-file"更进一步——把 file/stdin 读取抽成共享 read_text_file helper、显式区分 --stdin 与 --file -、三选一在 clap 层就锁死,测试也覆盖了你自己踩的那个多行反引号场景。env -u SANTI_WEBHOOK_* 那个干净环境的判断尤其到位。

一个顺带的观察,想听你的判断:santi inbox seed <text> 也是个 positional text: String,和 im reply 改之前一模一样的多行/引号摩擦——它同样是 offline store-writer,自升级流程 seed "come look" 记录时也可能带多行内容。要不要把你这次抽出来的 read_text_file helper 顺手也用到 inbox seed(同样加 --file/--stdin)?可以这个 PR 里一起,也可以留个 follow-up,你来定。

不急,按你的节奏。

[LiberteCode]

我判断这个和 im reply 是同一类摩擦，而且 inbox seed 也是 offline store-writer，不会扩大运行时边界，所以我直接放进这个 PR 里一起做了。

已追加：

• santi inbox seed <text> 保持兼容；
• 新增 santi inbox seed --file <PATH> / --file - / --stdin；
• 与 im reply 共享同一套 text-source 解析和 file/stdin reader；
• 补了 inbox seed 的 clap 互斥/必选测试，以及 file-source 读取覆盖。

我已 push 新提交 abbd6fc，并把 PR 标题/说明更新成覆盖两个 offline CLI text 入口。验证重新跑过：

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• env -u SANTI_WEBHOOK_GITHUB_ALLOW -u SANTI_WEBHOOK_GITHUB_LOGIN cargo test --locked --workspace